W.3F 


UNIVERSITY  OF  CALIFORNIA. 


Mrs.  SAfeAH  P.  WALS\VORTH. 

Received  October^  1894. 
Accessions  No.  &*7  %.      Class  No. 


HIGH-SCHOOL  ASTRONOMY 


IN  WHICn  THB 


DESCRIPTIVE,  PHYSICAL,  AND  PRACTICAL 


ARE    COMBINED, 


WITH   BPECIAL   REFERENCE.  TO  THE  WANTS   OV 


ACADEMIES  AND  SEMINARIES  OF  LEARNING. 


BY  HIRAM  MATTISON,  A.  M., 

LATE  PROFESSOR  OF  NATURAL  PHILOSOPHY  AND  ASTRONOMY  IN  THE  FAU,ST 

SEMINARY  ;  AUTHOR  OF  THE  PRIMALiY  ASTRONOMY  ;  ASTRONOMICAL 

MAPS;   EDITOR  OF  BURRITT'S  GEOGRAPHY  OF  THE 

UEAVENS,  ETC.,  ETC. 


%    506   J3ROA 

LOSTOX:    154  TEEMONT   ST. 
CHICAGO:    CEO.  &  C.  W.  SHEEY70GD. 


Enterod  according  to  Act  of  Congress,  In  tho  year  1858. 
BY    HIPvAM   MATT1SON, 

In  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the  Southern 
District  of  New  York. 


PREFACE. 


THE  design  of  this  work  is  to  furnish  a  suitable  text-book  of 
Astronomy  for  academies  and  seminaries  of  learning. 

For  juvenile  learners,  the  "Primary  Astronomy"  is  all  that 
can  be  desired ;  and  for  advanced  classes,  who  wish  to  study  the 
Constellations,  in  connection  with  Mythology,  the  "Geography  of 
the  Heavens  "  should  be  chosen  in  preference  to  all  others ;  but 
for  all  ordinary  students,  this  intermediate  work  will  be  found 
sufficiently  elementary  on  the  one  hand,  and  sufficiently  extended 
on  the  other. 

The  work  is  now  divided  into  three  parts.  After  an  introduc- 
tion, which  consists  of  Preliminary  Observations  and  Definitions, 
and  occupies  twenty  pages,  Part  First  is  devoted  to  the  Solar 
System — the  sun,  planets,  comets,  eclipses,  tides,  &c. ;  Part 
Second  relates  to  the  Sidereal  Heavens — the  fixed  stars,  con- 
stellations, clusters,  and  nebulae ;  and  Part  Third  to  Practical 
Astronomy — the  structure  and  use  of  instruments,  refraction, 
parallax,  &c.  This  department,  so  seldom  introduced  into  text- 
books for  schools,  will  be  found  especially  interesting  and  valu- 
able. 

Besides  embracing  all  the  late  discoveries  in  astronomy,  under 
a  strictly  philosophical  classification,  the  work  is  thoroughly 
illustrated,  by  the  introduction  of  diagrams  into  its  pages,  in  con- 
nection with  the  text;  and  the  adaptation  throughout  to  the  use 
of  the  black-board,  during  recitation,  cannot  fail  to  be  appreciated 
by  every  practical  teacher. 

The  variety  of  type  affords  an  agreeable  relief  to  the  eye  of  the 
student,  and  at  the  same  time  distinguishes  the  main  text  from 
the  less  important  matter,  the  more  careful  study  of  which  may 
be  left  for  a  review.  The  suggestive  topical  questions  at  the  bot- 
tom of  the  page  complete  the  design. 

On  the  whole,  the  work  is  believed  to  be  a  decided  improve- 
ment upon  the  works  heretofore  in  use  in  this  department  of 
study ;  and  as  such  it  is  offered  to  the  professional  teachers  of 
the  country. 

H.  MATTISOIT. 
New  York,  August,  1866. 


ASTRONOMICAL  WORKS 

In  the  Author' 9  JAbrary,  and  more  or  less  consulted  in  the  compilation 
of  the  following  pages : 

A  Cycle  of  Celestial  Objects,  for  the  use  of  Naval,  Military,  and  Private  Astronomer^ 

&c.    By  CAPT.  WM.  HENRY  SMYTH,  &<x    2  vols.  8vo.    London,  1844. 
An  Introduction  to  Astronomy,  in  a  Series  of  Letters  from  a  Preceptor  to  his  PupH, 

&c.    By  JOHN  BONNYCASTLE,  Professor  of  Mathematics,  &c.    1  vol.  8vo.    London, 

1822. 
An  Introduction  to  the  True  Astronomy;  or,  Astronomical  Lectures  read   in  the 

Astronomical  School  of  the  University  of  Oxford.    By  JOHN  KKILL,  M.  D.,  F.  11.  b., 

&c.     I  vol.  8vo.    Dublin,  1793. 
Astronomy  Explained,  upon  Sir  Isaac  Newton's  principles,  &c.,  &c.    By  JAMES  FEE- 

eu&ON,  F.  K.  S.    1  vol.  4to.    London,  1764 
The  Elements  of  Physical  and  Geometrical  Astronomy.    By  DAVID  GREGORY,  M.  D^ 

late  Sullivan  Professor  of  Astronomy  at  Oxford,  <fee.    2  vols.  8vo.     London,  1726. 
Astronomy,  in  Five  Books.    By  EOGEE  LONG,  D.  D.,  F.  K.  S.,  &c.,  University  of  Can> 

bridge.    2  vols.  4to.     Cambridge  (Eng.),  1742. 
Astronomia  Carolina,  &c.,  by  THOMAS  STREET;  and  A  Series  of  Observations  on  tha 

Planets,  chiefly  the  Moon,  &c.,  by  Du.  EDMUND  HALLEY.    1  vol.  4to.    London,  1716. 
Astronomical  Lectures,  read  in  the  Public  School  at  Cambridge  (Eng).    By  WILLIAM 

WHISTON,  M.  A.,  Professor  of  Mathematics,  &c.    1  vol.  Svo.    London,  1728. 
The  Wonders  of  tJie  Heavens, ;  a  popular  view  of  Astronomy,  &c.    By  DCNGAN  BSAD- 

FORD.    1  vol.  royal  4to.    New  York,  1843. 
Popular  Lectures  on  Science  and  Art,  &c.    By  DIONYSIUS  LARDNKB,  F.  E.  S.,  &c^ 

Ac.    2  vols.  Svo.    New  York,  1846. 
Outlines  of  Astronomy.    By  SIK  JOHN  F.  "W.  HEP.SCHEL,  Bart,  K.  H.,  &c.    1  vol.  Svo. 

Philadelphia,  1849. 
Pheiwmena  and  Order  of  the  Solar  System,  and  Views  »f  the  Architecture  of  ffie 

Heavens.    By  J.  P.  NICHOL,  F.  E.  S.  E.,  &c.    2  vols.  12mo.    New  York,  1842. 
TJie  Practical  Astronomer,  &c.    By  THOMAS  DICK,  LL.D.    1  vol.  12mo.    New  York, 

1846.    Also,  "  Celestial  Scenery,1'  and  "  The  Sidereal  Heavens,"  by  the  same  author. 
TJie  Planetary  and  Stellar  Worlds.    By  PKOF.  O.  M.  MITCHEL.    1  vol.  12mo.    New 

York,  1S49. 
An  Elementary  Treatise  on  Astronomy,  &c.    By  WILLIAM  A.  NOP.TON,  A.  M.    1  vol. 

Svo.    New  York,  1845. 
An  Introduction  to  Astronomy,  &c.    By  DENISON  OLMSTED,  A.  M.    1  vol.  Svo,    New 

York,  1844.    Also,  Letters  on  Astronomy,  and  Life  and  Writings  of  JSbenezer  Por- 
ter Ma-son,  by  the  same  author.    2  vols.  12mo. 
77*5  Solar  System;  or,  the  Sun,  Moon,  and  Stars.    By  J.  E.  HINP, Director  of  Mr. 

Bishop's  Observatory,  Eegcnt's  Park,  London.    1  vol.  12mo.     London,  1S52. 
A  Pictorial  Display  of  the  Astronomical  Phenomena  of  the  Universe,  &c.    By  C.  F. 

BLOPNT.    4to.    New  York,  1844. 
The  Recent  Progress  of  Astronomy,  &c.    By  ELIAS  Loosiis,  Professor  of  Mathematics, 

&c.    1  vol.  12mo.    New  York,  1850. 

dnnual  of  Scientific  Discovery,  &c.    By  DAVID  A.  WELLS,  A.  M.    1  vol.  12mo.    Bos- 
ton, 1852. 
T!ie  Sidereal  Messenger  ;  a  Monthly  Journal,  devoted  to  Astronomical  Hcienee.    By 

O.  M.  MITCBEL,  A.  M.    (Now  discontinued.) 

Also,  Astronomical  Lectures  by  ARAGO,  LARDNEK,  MITCHEL,  and  NIOHOI.;   and  Ele- 
mentary Treatises  by  BURRITT,  KENDAL,  UAKTLFT,  M.!!NTIRK.  AKBOTT.  OSTRANDF.R, 

BLAKE,  HASLKK,  SMITU,  CJLAKK,  vosK,TvuiK,  COU&TOCK,  HASKIN^,  UYAJS, 

KEATU, 


CONTENTS. 


IITBODUGTIOI^ 

PRELIMINARY   OBSERVATIONS   AND   DEFINITIONS. 

CHAP.  I. — ORIGIN  AND  HISTORY  OF  THE  SCIENCE. 

Ptolemaic  Theory  of  the  Structure  of  the  Universe .....  12 

The  Copernican  System 13 

II. — DEFINITIONS. 

Solids,  Surfaces,  Ac 16 

Spheres,  Hemispheres,  and  Spheroids 17 

Lines  and  Angles 19 

Of  Triangles 20 

Circles  and  Ellipses 21 

The  Terrestrial  Sphere 23 

The  Celestial  Sphere ' 25 

First  Grand  Divisions  of  the  Universe. .  28 


PART  FIRST. 

THE     SOLAR    SYSTEM. 

CHAP.  I. — THE  PRIMARY  PLANETS. 

Classification  of  the  Solar  Bodies 29 

Names  of  tho  Primary  Planets 31 

Explanation  of  Mythological  Signs 32 

Distances  of  the  Planets 36 

Light  and  Heat  of  the  Planets 38 

Magnitude  of  the  Planets. 40 

Density - 41 

Gravitation 42 

Periodic  Revolutions  of  the  Planets.  ...  . .  ^  1 


8  CONTENTS. 


PAG* 

CHAP.  I. — Hourly  Motion  of  the  Planets  in  their  Orbits 45 

Centripetal  and  Centrifugal  Forces 45 

Laws  of  Planetary  Motion 46 

Aspects  of  the  Planets 48 

Sidereal  and  Synodic  Revolution. 49 

The  Ecliptic,  Zodiac,  Signs,  Ac. . . .  : 50 

Celestial  Latitude  and  Longitude 53 

Mean  and  True  Places  of  a  Planet 54 

Direct  and  Retrograde  Motions 55 

Morning  and  Evening  Stars 67 

Deviation  of  the  Orbits  of  the  Planets  from  the  Ecliptic  58 

Philosophy  of  Transits 60 

IL — PRIMARY  PLANETS  CONTINUED. 

Inclination  of  the  Axis  of  the  Planets,  and  its  Effects. .  65 

Rotation  of  the  Planets  upon  their  axes 69 

Time 70 

Equation  of  Time 72 

Time,  as  affected  by  Longitude 76 

True  Figure  of  the  Planets 77 

Precession  of  the  Equinoxes 80 

HL — TELESCOPIC  VIEWS  OF  THE  PLANETS. 

Mercury — Phases,  Mountains,  <fcc 83 

Venus — Phases,  Mountains,  Atmosphere 84 

Mars—"  Continents  and  Seas,"  Color,  Snow  Banks 86 

The  Asteroids — Color,  Hazy  Appearance 87 

Jupiter — Oblateness,  Belts,  Moons 88 

Saturn — Oblateness,  Rings,  Belts,  Phases,  Moons 89 

Uranus — a  Telescopic  World,  Satellites 94 

Neptune — purely  Telescopic,  Satellite 91 

Herschel's  Solar  System  in  Miniature 91 

^ 

IV. — SEASONS  OF  THE  DIFFERENT  PLANETS,  Ac. 

Cause  of  the  Seasons — Mercury,  Earth 91 

Venus,  Mars,  Jupiter 98 

Saturn  and  Uranus 99 

Discovery  of  the  different  Planets 100 

V. — SECONDAKT  PLANETS — THE  Moow. 

Character  and  Number  of  the  Secondaries 102 

The  Moon's  Distance,  Shape,  Position  of  Orbit,  «fec 103 


CONTENTS. 


KK3U 

CHAP.  V.-  -Magnitude,  Density,  Revolution  East-ward 1G5 

Form  of  Lunar  Orbit ^ 107 

Cause  of  the  Moon's  Changes 109 

Natural  appearance — Same  side  always  toward  us. ...  Ill 

Moon's  Librations — in  Latitude  and  Longitude 112 

Telescopic  Appearance  of  the  Moon — Lunar  Mountains  1 V3 

Finding  the  Longitude  by  the  Moon's  place 115 

VL — ECLIPSES  OF  THE  SUN  AND  MOON. 

Philosophy  of  both 116 

Law  of  Shadows 117 

Why  not  two  Eclipses  every  Lunar  Month 118 

Why  Solar  pass  eastward  over  the  Sun,  and  Lunar  west- 
ward over  the  Moon 119 

Ecliptic  Limits — Umbra  and  Penumbra 120 

Why  all  Central  Eclipses  not  total 122 

VII. S  VTELLITES  OF  THE  EXTERIOR  PLANETS. 

Satellites  of  Jupiter — Distances,  Periods,  &c 124 

Eclipses  of  Jupiter's  Moons — Immersions  and  Emersions  1 26 

Moons  of  Saturn — Why  seldom  eclipsed 127V 

Satellite  of  Neptune 129 

VIII. — NATURE  AND  CAUSE  OF  TIDES. 

Description  of  Tides,  Causes,  <fcc ISO 

Spring  and  Neap  Tides '. 134 

*    •  ' 
IX. — OF  COMETS. 

Niunrs  Parts,  Orbits,  Ac. 136 

Magnitudes,  Velocity,  Temperature,  Periods 140 

Numbers,  Physical  Natures,  <fec 143 

X.— THE  SUN. 

True  Figure,  Spots 146 

Physical  Constitution,  Temperature 151 

Zodiacal  Light 153 

Sun's  Proper  Motion  in  Space 155 

XL — MISCELLANEOUS  REMARKS  UPON  THE  SOLAR  SYSTEM. 

Nebular  Theory  of  its  Origin 156 

Were  the  Asteroids  originally  one  Planet  ? 159 

Are  the  Planets  inhabited  by  rational  beings  ? 101 


CONTENTS. 


PART  SECOND. 

THE      SIDEREAL      HEAVENS. 

PACT 

CUAP.  I  —  THE  FIXED  STAUS. 

Classification  of  the  Stars  ..........................  166 

Number  of  the  Stars  ..............................  168 

Distances  of  the  Stars  .............................  170 

II  —  DESCRIPTION  OF  THE  CONSTELLATIONS. 

Nature,  Origin,  Classification  ........................  172 

Visible  in  October,  November,  and  December  .........  174 

"        January,  February,  and  March  .............  177 

"         April,  May,  and  June  .....................  180 

"        July,  August,  and  September  ..............  183 

III.  —  DOUBLE,  VARIABLE,  AND  TEMPORARY  STARS,  <fec. 

Stars  Optically  and  Physically  Double  ...............   187 

Binary  Systems  ..........................  ,  .......   189 

Variable  or  Periodical  Stars  ........................   194 

Temporary  Stars  —  New  and  Lost  ...................   190 


IV.  —  CLUSTERS  OK  STARS  AND 

Pleiades,  Hyades,  Ac  ..............................  199 

Nebulae  —  Resolvable,  Irresolvable,  Annular,  &c  ........  201 

Planetary,  Stellar,  <fec  ..............................  203 

Star  Dust,  Milky  Way  .............................  206 

PART  THIRD. 

PRACTICAL     ASTRONOMY. 

I.  —  PROPERTIES  OF  LIGHT. 

Refraction  of  Light  ...............................  211 

Atmospherical  Refraction  ..........................  214 

Refraction  by  Glass  Lenses  .........................  216 

II.  —  TELESCOPES. 

Refracting  Telescopes  .............................  221 

Reflecting  Telescopes  .............................  231 

Transit  Instrument  ................................  235 

Mural  Circle  .....................................  236 

Parallax  .........................................  2?37 

Meteors  and  Meteoric  Stones  .......................  239 

III.  —  PilOBLEMS   AND   TABLES  ......................  .     241 


INTRODUCTION. 


PRELIMINARY  OBSERVATIONS  AND  DEFINITIOJ 


CHAPTER   I. 

ORIGIN  AND  HISTORY  OF  THE  SCIENCE. 

1.  SCIENCE  is  knowledge  systematically  arranged,  so 
as  to  be  conveniently  taught,  easily  learned,  and  readily 
applied. 

2.  ASTRONOMY  is  the  science  of  the  heavenly  bodies — 
the  Sun,  Moon,  Planets,  Comets,  and  Fixed  Stars. 

The  term  astronomy  is  from  the  Greek  astron,  a  star,  and  nomo&,  a  law ;  and  sig- 
pifios  the  laws  or  science  of  the  stars. 

3.  Astronomy  is  divided  into  Descriptive,  Physical, 
and  Practical. 

Descriptive  Astronomy  includes  the  mere,  facts  of  the 
science,  irrespective  of  the  causes  of  the  phenomena  ob- 
served, or  of  the  means  by  which  the  facts  were  ascer- 
tained. 

Physical  Astronomy  explains  the  causes  of  the  vari- 
ous phenomena  observed,  as  of  Day  and  Night,  the 
Seasons,  Eclipses,  Tides,  &c. 

Practical  Astronomy  relates  to  the  means  for  acquiring 
astronomical  knowledge  by  the  use  of  instruments,  and 
by  mathematical  calculations. 

These  three  departments  have  arisen,  one  after  the  other,  in  the  order  in  which  they 
are  here  stated.  At  first  a  few  facts  and  phenomena  were  observed,  but  the  causes 
were  unknown.  Next  some  of  the  causes  were  investigated  one  by  one ;  and,  finally, 
instruments  were  invented  for  measuring  distances,  altitudes,  &c. ;  data  for  cou/culOr- 
tions  were  obtained;  and  thus  arose  the  department  of  Practical  Astronomy. 

1.  Define  the  term  Science. 

2.  What  is  Astronomy  1     (From  what  is  the  term  derived  ?) 

3.  How  is  astronomy  divided ?    Descriptive?    Physical?    Practical?  CStat;) 
tlio  order  n  which  these  departments  have  arisen.) 


10  ASTTOXOMY. 


4-.  Astronomy  lias  long  been  regarded  as  the  most 
sublime  of  the  sciences,  eminently  calculated  to  illua 
trate  the  wisdom,  power,  and  goodness  of  God  ;  to  ele- 
vate and  expand  the  human  mind,  and  to  fill  it  with  ex- 
alted views  of  the  Creator  — 

"  The  glorious  Architect  who  built  the  skies." 

1.  "The  greatest  men  of  all  ages  have  pronounced  this  science  to  he  the  most  ST.bumt 
end  snrpasfinjr  of  all  that  can  be  tested  by  human  genius,  and  to  be  worthy  of  a  life  of 
Btady.<"-iSm$&1«  CelvxtiaJ.  Cycle. 

2.  "Onr  very  faculties  are  enlarged  with  tlie  grandeur  of  the  ideas  it  conveys,  our 
minds  exalted  above  the  low-contracted  pnjudlces  of  the  vulgar,  and  our  understand- 
ings clearly  convinced,  and  affected  with  the  conviction  of  the  existence,  wisuom,  |>o\ver, 
goodness,  ar.tl  superintendencv  of  the  SUPREME  BEING!"  —  Ferguson. 

3.  So  remarkably  does  this  science  exhibit  the  glory  and   majesty  of  God,  by  its 
astounding  revelations  of  liis  works,  that  it  almost  necessarily  tends  to  fill  the  rniiid 
with  awe  and  reverence.    It  was  in  view  of  this  tendency  that  the  poet  Young  said, 

"  An  undevout  astronomer  i»  mad." 

4.  To  the  moral  influence  of  the  contemplation  of  the  heavers,  we  have  frequent 
reference  in  the  sacred  Scriptures.     "The  heavens  declare  the  gto.y  of  God;   and  the 
firmament  showeth  his  handy-work."    (Psalm  xix.  1.)    ""When  I  consider  thy  heavens, 
the  work  of  thy  fingers;  the  moon  and  stars,  which  tliou  hast  ordained  ;  what  is  man, 
that  thou  art  mindful  of-  him?  and  the  son  of  man,  that  thou  visitest  him?"    (Psalm 
viii.  8,  4.) 

5.  Astronomy  is  probably  the  most  ancient  of  all  the 
sciences.  Some  of  the  Chaldean  observations  date  as 
far  back  as  2,250  years  before  Christ,  or  only  98  years 
after  the  Flood  !  Laplace  speaks  confidently  of  Chinese 
observations  1,100  r>.  c.  ;  and  Mr.  Bailly,  an  English 
astronomer,  fixes  the  time  of  a  conjunction  of  Mars, 
Jupiter,  Saturn,  and  Mercury,  mentioned  in  Chinese 
records,  at  2,M9  years  before  Christ. 

1.  The  ancient  Chinese  astronomers  and  mathematicians  were  held  to  a  fearful  re- 
fponsibility  for  the  correctness  of  their  calculations.     In  the  reign  of  the  Emperor  (Jhou- 
UTig,   his  two  chief  astronomers,  ITo  and  //?,  were  condemned  to  death  for  neglecting  to> 
announce  the  precise  time  of  a  solar  eclipse,  which  took  place  B.  C.  2,169. 

2.  The  Holy  Scriptures,  some  parts  of  which  are  very  ancient,  contain  several  allusions 
to  the  science  of  astronomy.     In  the  first  chapter  of  Genesis  we  have  an  account  of  the 
crfation  of  tlie  Sun,  Moon,  and  Stars.    "And  God  said,  Let  there  be  lights  in  the  firma- 
rrent  of  the.  heaven,  to  divide  the  day  from  the  night,  and  let  them  be  for  signs,  and  for 
seasons,  and  for  days  and  years.     And  let  them  be  for  lights  in  the  firmament  of  tlie 
heaven,  to  give  light  upon  the  earth:  and  it  was  so.     And  God  made  TWO  great  lights; 
the  greater  light  to  rule  the  day,  and  the  lesser  light  to  rule  the  night:  he  made  the 
et.irs  also."    Verses  14-16. 

3.  In  the  book  of  Job,  written  1,50r-  years  before  Christ,  we  read  of  several  constella- 
tions that  bear  tlie  same  names  now  tn'at  they  did  three  thousand  years  ago.     "Which 
maketh  Arcturns.  Orion,  and  Pleiades,  and  the  chambers  of  the  south."  (ixT  9.)    Again  . 
'  Canst  thou  bind  tlie  sweet  influences  of  Pleiades,  or  loose  the  bands  of  Orion  ?    Canst 
ti'ou  bring  forth  Mazzaroth  in  his  season?  or  canst  thou  guide  Areturus  with  his  sons?" 

xxx  viii.  81,  82.) 


4.  How  astronomy  regarded  ?    (Smyth?  Ferguson?    Young?    Scriptures?) 
o.  What  of  antiquity  of  astronomy'?     Chaldean  and  Chinese  observations  i 
(Responsibility  of  Chinese  astronomers  ?    Ancient  Scriptural  allusions  $) 


EARLY    ASTRONOMERS. 


11 


6.  The  first  astronomers  were  shepherds  and  herds7/ien^ 
who  were  led  to  this  study  by  observing  the  movements 
of  the  sun,  moon,  and  stars,  while  watching  their  flocks 
from  year  to  year  in  the  open  fields. 

ANCIENT   ASTRONOMERS   OBSERVING   THE   IIEAVEM5. 


7.  TJiales,  one  of  the  seven  wise  men  of  Greece,  was 
the  first  regular  teacher  of  Astronomy,  B.  c.  600.     The 
next  was  Anaximander,  a  disciple  of  Thales,  who  suc- 
ceeded him  as  head  of  the  school  at  Miletus,  B.  c.  548. 
lie  asserted  the  true  figure  of  the  earth,  and  seems  to 
have  had  some  idea  of  its  daily  re-volution. 

Anaximanrler  is  supposed  to  have  been  the  first  who  constructed  globes  and  mape. 
He  taught  that  the  moon  shines  by  reflection,  and  in  several  other  respects  advanci-d 
beyond  the  knowledge  imparted  by  his  distinguished  tutor. 

8.  Pythagoras,     another    Greek     philosopher,    who 
founded  the  school  of  Croton,  B.  c.  500,  greatly  enlarged 
the  science.     He  first  gave  form  to  the  vague  ideas  that 
the  sun  was  in  the  center  of  the  planetary  orbits,  that 
the  earth  floated  unsupported  in  space,  and  that  the  dis- 
tant stars  were  worlds,  and  probably  inhabited. 


. 

jtcturos  of  a  sagacious  mind,  not  possessed  of  the  evidence  requisite  to  gi 
it,5  opinions,"    Pythagoras  is  said  lo  nave  perished  from  hunger,  in  his  old 


"  It  was  Pythagoras,1"  says  Smyth,  "  who  taught,  in  fact,  the  system  which  now  im- 
mortalizes the  name  of  Copernicus."    But  lie  adds  that  li  is  teach  ings  were  but  "the  con- 

quisite  to  give  stability  to 
age. 

6.  Who  were  the  first  astronomers  ?     How  led  to  this  study  ? 

7.  Who  first  regular  teacher  of  this  science  ?     How  early?     Who  next  ?— 
and  when  ?     What  correct  notions  did  lie  seem  to  entertain  ?     (For  what  elso 
didting'ushed  ?) 

8.  "Who  next  after  Anaximandcr  ?    -What  advances  did  he  make  in  this 
Study  ?     (What  does  Smyth  sav  of  his  teachings?     What  said  of  his  death  f 


12 


AbTRONOMY. 


9.  Ptolemy,  an  Egyptian  philosopher,  taught  astronomy 
in  the  second  century  of  the  Christian  era.  He  adopted 
the  theory  that  the  earth  was  located  in  the  center  of  the 
universe,  that  it  was  perfectly  at  rest,  and  that  the  sun, 
moon,  and  stars  actually  revolved  around  it,  from  east  to 
west,  as  they  appear  to  do,  every  twenty-four  hours.  This 
system  is  called,  after  its  author,  the  Ptolemaic  Theory. 


PTOLEMAIC  THEORY  OF  THE  STRUCTURE  OF  THE  UNIVERSE. 


1.  Ptolemy  supposed  the  earth  to  be  in  the  center  of  a  system  of  crystalline  arches, 
or  hollow  spheres,  arranged  one  within  the  other,  as  represented  in  the  cut.  It  is  thought 
by  some  that  he  understood  the  spherical  figure  of  the  earth,  and  the  cut  is  constructed 
upon  this  supposition.  Ptolemy  further  supposed  that  the  sun,  moon,  and  stars  were 
fixed  in  these  crystalline  spheres,  at  different  distances  from  our  globe ;  that  the  Moon 
was  in  the  first,  Mercury  in  the  second,  Venus  in  the  third-  the  Sun  in  the  fourth.  Mars. 

9.  Who  was  Ptolemy? — and  when  did  he  flourish?  Describe  his  theory, 
(ilow  lorvite  sun,  moon,  &e.  ?  What  absurdity  did  it  involve,  ;w*  it  respects 


COPEENICAN   SYSTEM.  13 


Jupiter,  and  Saturn  in  the  next  three,  and  the  fixed  stars  in  the  eighth.  The  ancicnta 
had  no  knowledge  of  Uranus  or  Neptune.  This  ponderous  machinery  was  supposed  to 
revolve  from  eat-t  to  west  around  the  earth,  carrying  with  it  the  sun^  moon,  and  stars, 
every  twenty-four  hours;  and  the  spheres  being  crystal,  the  distant  stars  were  visible 
through  them. 

2.  If  the  sun  was  designed  to  enlighten  and  warm  the  different  sides  of  our  globe, 
the  Ptolemaic  method  of  effecting  this  object  is  most  unreasonable.    To  carry  the  sun 
around  the  earth,  to  warm  and  enlighten  its  different  sides,  instead  of  having  the  earth 
turn  first  one  side  and  then  the  other  to  the  sun,  by  a  revolution  on  its  axis,  would  be 
like  carrying  a  fire  around  a  person  who  was  cold,  and  wished  to  be  warmed,  instead  of 
his  turning  himself  to  the  fire  as  he  pleased. 

3.  The  Ptolemaic  theory  would  require  a  motion  inconceivably  rapid  in  all  the 
heavenly  bodies.     As  the  sun  is  ninety-five  millions  of  miles  from  the  earth,  the  entire 
diameter  of  his  sphere  would  be  one  hundred  and  ninety  millions  of  miles,  and  its  cir- 
cumference about  six  hundred  millions.    Divide  this  distance  by  twenty -four — the  num- 
ber of  hours  in  a  day — and  it  gives  twenty-five  'million  miles  an  hour,  or  sixty -nine 
thousand  four  hundred  and  forty-four  miles  per  second,  as  the  velocity  of  the  sun  !    This 
theory  would  require  a  still  more  rapid  motion  in  the  fixed  stars.     It  would  require  the 
nearest  of  these  to  move  at  the  rate  of  nearly  fourteen  thousand  millions  of  miles  per 
second,  or  seventy  thousand  times  as  swift  as  light,  in  order  to  accomplish  their  daily 
course.    But  with  all  these  difliculties  in  its  way,  the  Ptolemaic  theory  was  generally 
believed  till  about  the  middle  of  the  sixteenth  century,  or  three  hundred  years  ago. 

THE   COPERNICAN   SYSTEM. 

10.  About  the  year  1510,  the  ancient  theory  of  Pythag- 
oras was  revived  and  improved  by  Copernicus,  a  Prus- 
sian astronomer,  and  has  since  been  called,  after  him,  the 

Copernican  System. 

1.  The  investigations  of  Copernicus  were  conducted  between  the  years  1507  and  1530. 
In  the  latter  year  he  finished  his  tables  of  the  planets,  and  his  great  work,  The  Revolu- 
tion of  the  Celestial  Orbs  ;  but  he  did  not  venture  to  publish  his  views  till  thirteen  years 
after,  or  1543,  when  he  recer- " :<l  a  copy  of  it  only  a  few  hours  before  his  death,  and  con- 
sequently never  read  it  in  print.    It  contains  the  old  philosophy,  interspersed  with  his 
own  original  and  acute  conceptions,  and  was  received  under  very  considerable  opposi- 
tion.— Smyth,  vol.  1,  p.  88. 

2.  Copernicus  is  generally  regarded  as  the  discoverer  of  the  system  which  bears  his 
name,  but  this  is  a  popular  error.     There  is  abundant  proof,  notwithstanding  the  loss  of 
his  writings,  that  Pythagoras  understood  the  leading  features  of  what  is  now  called  the 
Copernican  Theory. 

11.  The  first  prominent  feature  of  the  Copernican  sys- 
tem is,  that  the  earth  is  a  sphere  or  globe^  inhabited  on 

all  sides. 

The  evidence  that  the  earth  is  a  sphere  or  globe  may  be  arranged  and  stated  as  fol- 
iows : 

1.  Admitting  that  the  sur,  moon,  and  stars  are  worlds,  the  fact  that  they  are  round, 
as  we  see  them  to  be,  affords  ground  for  the  presumption,  at  least,  that  the  earth  also  is 
round, 

2.  Water  falling  from  the  clouds  is  gathered  into  little  globes  or  drops;  and  melted 
lead  poured  from  the  summit  of  a  high  tower  assumes  the  form  of  globes,  which,  when 
cooled,  are  called  shot.    And  the  same  law  would  cause  a  larger  mass  of  fluid  matter,  if 
loft  undisturbed  in  space,  to  assume  the  same  shape.    But  the  Bible  teaches  that  the 

light  and  heat  ?  What  in  respect  to  the  motions  of  the  heavenly  bodies  ? 
Was  such  a  theory  ever  .generally  believed  ?  Till  how  recently  ?) 

10.  Who  was  Copernicus?     for  what  distinguished  ?    About  what  time  ? 
(Whaf   of  his  investigations?     His  work?    Its  publication?    Character's 
What  popular  error  noticed  ?) 

11.  State  the  first  leading  feature   of    the  Copernican  theory.     (What 
proofs  of  its  correctness  ?    The  first  ?    Second?    Third?    Fourth?    Fifth? 
SLxth  I     Seventh  ?) 


14:  ASTRONOMY". 


whole  enrth  was  once  In  a  fluid  state — one  vast  drop — the  substances  now  constituting 
the  oceans  and  continents  being  Indiscriminately  mingled  together.  "And  the  earth 
was  without  form  and  void  [/.  />.,  chaotic,  confused,  unorganized],  and  darkness  dwelt 
upon  the  face  of  the  deep;  and  the  spirit  of  God  moved  upon  the  face  of  the  wnters. 
*  *  *  And  Go*  said.  Let  the  waters  under  the  heavens  be  gathered  together  unto 
one  placf,  and  let  ....vj  dry  land  appear:  and  it  was  so.  And  God  called  the  dry  land 
earth,,  and  the  gathering  together  of  the  waters  called  he  SKIS.*' — Genesis  i.  2,  9.  10.  Up 
to  this  time  there  was  no  "  earth,"  either  as  continents  or  islands,  neither  were  there  any 
"seas," but  all  the  elements  were  mingled  together;  and  a  mass  of  Uuid  thus  dropped 
iiito  space,  from  the  hand  of  the  Creator,  would  be  as  certain  to  assume  the  form  of  a 
globe,  as  the  melted  lead  from  the  shot-tower,  or  the  water  from  the  passing  cloud. 

3.  The  apparent  elevation  and  depression  of  the  North  Star,  as  we  approach  toward  or 
recede  from  it,  shows  that  the  surface  of  the  earth  is  convex,  or  that  the  earth  is  a  globe. 

4.  The  fact  that  the  tops  of  mountains  are  last  seen  as  we  recede  from,  or  first  as  wo 
approach,  the  sea-shore,  proves  that  the  surface  of  the  water  upon  which  we  sail  is  con- 
vex ;  so  when  a  ship  is  approaching  the  shore,  the  topmasts  are  always  seen  first,  and 
the  hull  or  body  last    And  when  seamen  wish  to  survey  the  horizon' at  sea  to  R  great 
distance,  in  search  of  whale  or  other  shipping,  they  "  go  to  the  mast-head,"  as  they  call 
it  from  which  point  they  can  often  discover  objects  that  are  entirely  invisible  from  tho 
deck,  of  the  ships. 

5.  If  an  aqueduct  is  to  be  constructed  a  mile  lonsr,  so  as  to  be  filled  with  water  to  tho 
brim  at  every  point,  it  must  be  about  eight  inches  higher  in  the  middle  than  at  the  ends, 
so  as  to  allow  the  surface  of  the  water  to  conform  to  the  convex  figure  of  the  globe.     We 
eay  higher,  not  that  it  needs  to  be  higher  as  determined  by  a  water  level,  for  a  water 
level  is  convex, but  higher  as  determined  by  a  straight  line  drawn  from  one  end  of  tho 
aqueduct  to  the  other.    This  definite  knowledge  of  the  curvature  of  water,  even  for  small 
distances,  shows  that  the  earth's  surface  is  convex — or,  in  other  words,  that  the  earth  ia 
spherical.    (The  curvature  Irom  a  tangent  line  is  8  inches  tor  one  mile,  trom  the  point 
of  contact;  32  inches  for  two  miles;  72  inches  for  three  miles,  &c.) 

6.  When  the  moon  falls  into  the  shadow  of  the   earth  and  is  eclipsed,  or,  in  othei 
words,  the  earth  gets  into  her  sunlight,  and  throws  its  shadow  upon  her,  the  shadow  is 
seen  to  be  convex.    We  must  either  conclude,  therefore,  that  the  earth,  which  casts  tho 
shadow,  is  in  the  form  of  a  dinner-pi  ate,  and  is  .always  kept  sidewise,  and  the  same  side 
toward  the  sun  (which  we  know  is  not  the  case) ;  or  that  it  is  a  globe,  and  casts  a  coni- 
cal shadow,  whatever  its  position. 

7.  The  earth  is  known  to  be  a  globe,  from  the  fact  tint,  ships  are  constantly  sailing 
around  it. 

8.  It  is  not  certain  whether  Ptolemy  admitted  the  earth  to  be  a  sphere  or  not.    Some 
writers  maintain  that  he  rejected  this  doctrine,  and  others  that  he  admitted  it     In  the 
"  PRIMARY  ASTRONOMY,"  page  8,  the  author  has  inserted  a  cut  representing  the  Ptole- 
maic theory,  with  the  earthy?a£;  but  in  this  work  (page  12),  where  the  same  theory  is 
represented,  the  earth  is  shown  as  a  globe.     In  all  other  respects,  the  theory  represented 
is  the  same  in  both  works;  and  this  is  only  a  minor  point  in  the  system. 

12.  A  second  leading  feature  of  the  Copernican  theory 
is,  that  the  apparent  revolution  of  the  sun,  moon,  and 
stars  westward  every  day,  is  caused  by  the  revolution  of 
the  earth  around  its  own  axis,  from  west  to  east,  every 
twenty-four  hours. 

That  the  heavenly  bodies  appear  to  revolve  westward,  is  no  proof  that  they  are  acta- 
ttlly  in  motion.  We  often  transfer  our  own  motion,  in  imagination,  to  bodies  that  are  at 
rest;  especially  when  carried  swiftly  forward  without  any  apparent  cause,  as  when  0:10 
travels  in  a  steamboat  or  railway  car,  and  when  for  a  time  he  forgets  his  own  motion. 
"  Copernicus  tells  us  that  he  was  first  led  to  think  that  the  apparent  motions  of  .the  heav- 
enly bodies,  in  their  diurnal  revolution,  were  owing  to  the  real  motion  of  the  earth  in 
the  opposite  direction,  from  observing  instances  »f  the  same  kind  among  terrestrial  ob- 
•ects ;  as  when  the  shore  seems  to  the  mariner  to  recede  as  he  rapidly  sails  from  it,  and 
as  trees  and  other  objects  seem  to  glide  by  us,  when,  on  riding  swiftly  past  them,  we  lose 
the  consciousness  of  our  own  motion."  This  remark  would  go  to  show  that  tue  revolu- 
tion of  the  earth  on  its  own  axis  was  an  original  discovery  with  Copernicus. 

12.  State  the  second  leading  feature  of  the  Coperuican  system.  (Do  not 
our  own  senses  furnish  proof  that  the  heavenly  bodies  revolve  westward 
daily  ?  Why  not  ?  What  remark  from  Copernicus?  What  does  it  seeui  to 
uiii'ly  3) 


COPEENICAN    SYSTEM. 


15 


13.  A  third  feature  of  the  Copernican  theory  is,  that 
the  sun  is  the  grand  center  around  which  the  earth  and 
all  the  other  planets  revoLyA:, 


THK    COPHRNICAN    SYSTEM. 


1.  The  above  cut  is  a  representation  of  the  Copernicam.  Theory  of  the  Solar  System. 
In  the  center  is  seen  the  sun.  in  a  state  of  rest.     Around  him,  at  unequal  distances,  are 
the  planets  and  fixed  stars — the  former  revolving  about  him  irom  -west  to  east,  or  from 
the  right  over  to  the  left.     The  white  circles  represent  the  orbits,  or  paths,  in  which  t!io 
planets  move  around  the  sun.     On  the  right  is  seen  a  comet  plunging  down  into  the  sys- 
tem around  the  sun,  and  then  departing.    This  is  the  Copernican  Tiieory  of  the  Solar 
System. 

"  Ohow  unlike  the  complex  works  of  man, 
Heaven's  easy,  artless,  uneucumber'd  plan  !" 

2.  The  truth  of  the  Copernican  theory  is  established  by  the  most  conclusive  and  satis- 
factory evidence.     Eclipses  of  the  sun  and  moon  are  calculated  upon  this  theory,  and 
astronomers  are  able  to  predict  thereby  their  commencement,  duration,  Ac.,  to  a  minute, 
even  hundreds  of  years  before  they  occur.    "We  shall  therefore  assume  the  truth  of  this 
system  without  further  proof,  as  we  proceed  hereafter  to  the  study  of  the  heavenly 
bodies. 

18.  State  the  third  prominent  feature  of  the  theory  of  Copernicus.  (De- 
scribe the  cut.  AY  hat  additional  evidence  of  the  truth  of  tliia  theory,  as  a 
whole  0 


16  ASTRONOMY. 


CHAPTER    II. 

DEFINITIONS.* 

14.  SOLIDS,  SURFACES,  &c. 

A  Solid,  or  Body,  is  a  figure  having  length,  breadth, 
and  thickness. 

A  Surface  is  the  outside  or  exterior  of  a  body,  and  has 
length  and  breadth  only. 

Surfaces  are  of  three  kinds — Plane,  Concave,  and  Cw^ 
vex. 

A  surface  may  also  be  rough  or  smooth,  hard  or  soft ;  the  above  definition  having 
reference  only  to  the  general  jig  ur«  of  bodies. 

A  Plane  Surface  is  one  that  is  perfectly  flat  or  even, 
1'ke  the  floor  of  a  building,  or  the  sides  of  a  room. 

1.  We  may  imagine  what  is  called  a  plane,  to  extend  off  beyond  the  plane  surface* 
as  far  as  we  please  ;  or,  in  other  woi  ds,  to  be  indefinitely  extended.  When  a  plane  or 
ft  line  is  extended  in  this  way,  it  is  said  to  be  produced. 


2.  An  imaginary  plane  may  exist  where  there  is  no  body  having  a  plane  surface;  or 
between  two  lines,  like  the  plane  of  a  circle.  A  sheet  of  tin,  laid  across  a  small  wire 
hoop,  would  represent  the  plane  of  that  circle,  in  whatever  position  it  might  be  held. 


whether  horizontally,  perpendicularly,  or  otherwise;  and  the  place  which  the  tin  woul<i 
pass  through,  if  extended  to  the  starry  heavens,  is  the  plane  of  that  circle. 

3.  All  objects  which  the  tin  would  touch  or  cut,  if  extended  outward  to 
the  heavens,  or  to  infinity,  are  in  tlie  plane  of  the  sheet,  or  the  circle  upon 
which  it  is  laid.  A  point  is  in  a  plane  produced,  when  the  plane  continued 
or  extended  would  pass  through  that  point. 

Parallel  Planes  are  such  as  would  never  meet 
or  cut  each  other,  however  far  they  mi.»ht  be  ex- 
tended. 

The  two  sides  of  a  board,  or  two  sheets  of  tin  placed  equidistant  from  each 
other  at  every  point,  represent  parallel  planes. 


*  To  some  who  will  use  this  work,  many  of  the  following  diagrams  and  definitions  w'll 
be  superfluous,  the  substance  of  them  being  already  sufficiently  understood.  With  such 
students  the  judicious  teacher  will  pass  rapidly  over  the  next  ten  pages,  or  omit  them 
altogether. 


14.  Define  a  «>&/,  or  body  —  a  surface.  How  many  kinds  of  surfaces! 
(Any  other  distinctions?)  What  is  a  plane  surface?  (Maya  plane  extend 
beyond  the  plane  surface  ?  May  a  plane  exist  where  there  is  no  body  ?  II- 
lustrate.  What  is  a  plane  produced  ?)  What  are  parallel  planes  ? 


DEFINITIONS. 


17 


PERPENDICULAR  PLANES. 


Perpendicular  Planes  are 
such  as  stand  exactly  upright 
upon  each  other,  or  cross  each 
oilier  at  right  angles. 

In  the  figure,  one  plane  is  placed  horizontally, 
hml  the  other  perpendicular  to  it  They  arc 
therefore  perpendicular  to  each  other,  however 
they  may  stand  in  relation  to  the  observer. 

Inclined  Planes  are  such  as 
are  inclined  toward,  and  cut 
each  other  obliquely. 

The  Angle  of  Inclination  is 
the  angle  contained  between  the 
two  surl'aces  of  the  planes  near- 
est each  other. 

Trie  spaces  A  and  B  in  the  adjoining  cut  repre- 
sent the  Angle  of  Inclination. 

The  Area  of  a  plane  figure  is  the  amount  of  surface 
contained  therein. 


CONVEX  A1TD  CONCAVE  SCT-FACEO. 


A  Convex  Surface  is  one  that 
is  swollen  out  like  the  outside  of 
a  bowl. 

A  Concave  Surface  is  one  that 
is  hollowed  out  like  the  inside 
of  a  bowl. 

15.  SPHERES,  HEMISPHERES, 
and  SPHEROIDS. 


A  Sphere  is  a  globe  or  ~ball,  every 
part  of  the  surface  of  which  is  equidis- 
tant from  a  point  within,  called  its 
center. 

This  is  the  ordinary  definition;  but  in  Astronomy,  the  terra 
is  applied  to  the  apparent  concave  of  tho  heavens,  as  if  it  were 
the  actual  concave  surface  of  a  hollow  sphere. 


dicular  I     Inclined  ?     What  is  meant  by  the  angle  of  inclination  ?     The  ar&i 
of  a  plane  surface  ?     Describe  a  convex  surface — a  concave. 

15.  Describe  a  sphere — hemisphere — spheroH.    (Derivation  of  spheroid?) 


18 


ASTRONOMY. 


A  HEMISFHEB 


AN  OBLATE  SPHEROID. 


A  Hemisp7iere  is  the  Jialf  of  a  sphere  or 
globe,  or  of  the  apparent  concave  of  the  heav- 
ens. 

In  Geography  we  often  read  of  the  Eastern  and  Western,  and  North- 
ern and  teouthern  hemispheres,  but  in  Astronomy  the  term  is  only  ap- 
plied to  the  Northern  and  Southern  portions  of  the  heavens. 


A  Spheroid  is  a  body  resembling  a  sphere,  but  yet 
not  perfectly  round  or  spherical. 

The  term  spheroid  is  from  the  Greek  sphaira,  a  sphere,  and  eidos,  form,  and  signi 
fies  sphere-like. 

Spheroids  are  of  two  kinds — Oblate, 
and  Oblong  or  Prolate. 

An  Oblate  Spheroid  is  a  globe 
slightly  flattened,  as  if  pressed  on  oppo- 
site sides. 

This  is  a  difficult  figure  to  represent  upon  paper.  Should 
the  pupil  fail  to  obtain  a  correct  idea,  the  Teacher  will  be  at 
no  loss  for  an  illustration. 

A    Prolate 
sphere. 

This  figure,  like  an  Oblate  Spheroid,  admits  of  various  degrees  of  departure  from  the 
spherical  form.  It  may  be  much  or  but  slightly  elongated,  and  the  ends  may  be  alike  or 
other\vise.  A  common  egg  is  an  Oblong  Spheroid. 


or    Oblong    Spheroid  is    an   elongated 


AXIS  OF  A  SPHERE. 


The  Axis  of  a  sphere  is 
the  line,  real  or  imaginary, 
around  which  it  revolves. 

The  Poles  of  a  sphere 
are  the  extremities  of  its 
axis,  or  the  points  where 
the  axis  cuts  the  two  op- 
posite surfaces. 

The  Equator  of  a  sphere  is  an  imaginary  circle  upon 
its  surface,  midway  between  its  poles,  the  plane  of  which 
cuts  the  axis  perpendicularly,  and  divides  the  sphere 
Into  two  equal  parts  or  hemispheres. 

Kinds  of  spheroids?  Describe  each.  What  is  the  axis  of  a  sphere  ?  What 
the  pofa  /  The  equator  t  By  what  other  name  culled  ?  What  a  Less  Cir-jle  ? 
licndieiue  ? 


DEFINITIONS. 


19 


The  equator  of  a  sphere  is  sometimes 
called  a  Great  Circle,  because  no  larger 
circle  can  be  drawn  upon  its  surface. 

A  Less  Circle  is  one  that  divides  a 
sphere  into  two  unequal  parts. 


In  the  cut,  the  circles  are  re 


ve.    The 


represented  in  perspecti 
iddle  of  the  sphere,  whe 
diameter  is  included;  while  the  Less  Circle  passes  around  it 
between  the  Equator  and  the  Poles,  and  ia  consequently  "  less" 
than  the  Equator. 

Meridians  of  a  sphere  are  lines 
drawn  from  pole  to  pole  upon  its 
surface. 

16.  LINES  and  ANGLES. 


A  Point  is  that  which  has  no  magnitude  or  extension, 
but  simply  position. 

"  The  common  notion  of  a  point  is  derived  from  the  extremity  of  some  slender  body, 
such  as  the  extremity  of  a  common  sewing-needle.  This  being  perceptible  to  the 
senses,  is  a  physical  point,  and  not  a  mathematical  point;  SOT,  by  the  definition,  a 
point  has  no  magnitude." — PROFESSOR  PERKINS. 

A  Right  Line  is  the  shortest  distance 
between  two  points. 

A  Curved  Line  is  one  that  departs  con- 
tinually from  a  direct  course. 

Parallel  Lines  are  such  as  remain  at 
the  same  distance  from  each  other  through- 
out their  whole  extent. 

Oblique  Lines  are  such  as  are  not  paral- 
lel, but  incline  toward  or  approach  each 
other. 

When  two  lines  intersect  or  cut  each 
other,  the  space  included  between  them  is 
called  an  Angle. 


A  RIGHT  LINE. 


CUEVliD   LINE. 


PARALLEL  LINES. 


OBLIQTTE  LINES. 


16.  What  is  a  point  ?  (Physical  ?  Mathematical  ?)  A  right  line  ?— a  curved 
line  ? — parallel  lines  ? — an  angle  ? — kind  of  angles  f  Describe  u  rigl \  angle 
— an  acute — an  obtuse 


20 


ASTRONOMY. 


ACUTE  AND  OBTUbC 
A-NULKS. 


Angles  are  of  three  kinds — namely,  the  Right  Angle, 
the  Acute  Angle,  and  the  Obtuse  Angle. 

Right  Angles  are  formed  when  one 
right  line  intersects  another  perpendicu- 
larly, and  the  angles  on  each  side  are 
equal. 

An  Acute  angle  is  one  that  is  less,  and 
an  Obtuse  angle  one  that  is  greater,  than 
a  right  angle. 

17.  OF  TRIANGLES. 

A  Triangle  is  a   plane  figure,  bounded  by  straight 
lines,  and  having  only  three  sides. 

Triangles  are  of  six  kinds — viz.,  Right-angled,  Obtuse- 
angled,  Acute-angled,  Equilateral,  Isosceles,  and  Scalene. 

A  Right-angled  Triangle  is  one  having 
one  right  angle. 

The  parts  of  a  Bight-angled  Triangle 
are  the  Base,  the  Perpendicular,  and  the 
Hypothenuse. 


BIGHT- ANGLED 
TRIANGLE. 


BASE 


Ilypotlienuse,  from  a  Greek  word,  which  signifies  to  subtend  or  stretch — a  line  sub- 
tended from  the  base  to  the  perpendicular. 

OBTUSE- ANGLED  TRIANGLE. 


An  Obtuse  angled  Triangle  is 
one  having  an  obtuse  angle. 


An  Acute-angled   Triangle  is   one 
having  three  acute  angles. 


An  Equilateral  Triangle  has  all  three  of 
its  sides  equal. 

Equilateral,  from  the  Latin  ceqtvus,  equal,  and  lateralis,  from 
latus,  side. 


AN  EQUILATERAL 
TRIANGLE 


17.  What  is  a  triangle?     How  many  kinds  ?     Describe  (or  draw)  a  right- 
triangle.     Describe  its  parts.    (IIypotlienu.se  0    Au  obtuse  ?    Acute? 


DEFINITIONS. 


21 


An  Isosceles  Triangle  has  only  two  of  its 
sides  equal. 

The  term  Isosceles  is  from  a  Greek  word,  signifying  equal  legs; 
hence  a  triangle  with  two  equal  legs  is  called  an  isosceles  Triangle. 


A  Scalene  Triangle  is  one  having  no  "two  sides 
equal. 

The  term  Scalene  is  from  the  Greek  skalenos,  and  signifies  obliquo,  unequal.  (See 
obtuse  and  acute  angled.) 

A  CIKCLK. 

18.  CIRCLES  AND  ELLIPSES. 

A  Circle  is  a  plane  figure,  bounded  by  a 
curved  line,  every  part  of  which  is  equally 
distant  from  a  point  within  called  the  center. 

Concentric  Circles  are  such  as  are  drawn 
around  a  common  center. 

The  Circumference  of  a  circle  is  the  curved 
line  which  bounds  it. 

The  Diameter  of  a  circle  is  a  right 
line  passing  through  its  center,  and  ter 
minating  each  way  in  the  circumfer- 
ence. 

The  Radius  of  a  circle  is  a  right  line 
drawn  fom  its  center  to  any  point  in  the 
circumference. 

The  plural  of  radius  is  radii ;  and  as  radii  proceed  from  a  common  center,  light, 
which  proceeds  from  a  luminous  point  in  all  directions,  is  said  to  radiate ;  and  the 
'ight  thus  dispersed  is  sometimes  called  radiations  or  radiance. 

All  circles,  whether  great  or  small,  are  supposed  to  be 
divided  into  360  equal  parts,  called  degrees;  each  degree 
into  60  equal  parts,  called  minutes y  and  each  minute 
into  60  equal  parts,  called  seconds.  They  are  marked 
respectively  thus :  Degrees  (°),  minutes  ('),  seconds  ("). 

Equilateral?    (Derivation?)    Isosceles?     (Derivation?)    Scalene?    (Derivjk- 
tion  ?) 

18.  What  is  a  circle?  Concentric  circl  es  ?  The  Circumference  ?  Dimeter* 
Rudris2  (I'lurai,  &c.?)  How  all  circles  divided ?  (What  is  a  j.wtractor* 


DIAMETER,  CIRCUMFKlt- 
ENCE,  ETC. 


ASTRONOMY. 


A  PROTRACTOR. 


PARTS  OF  A  CIRCLE. 


To  feave  the  trouble  of  dividing  a  circle  into  860°,  in  order  to  measure  the  degrees 
of  an  angle,  we  make  use  of  an  instrument  called  a  Protractor.  It  consists  of  a  semi- 
circle of  silver  or  brass,  divided  into  de- 
grees, as  represented  in  the  inclosed  figure. 
To  measure  an  angle,  as  A  B  C,  the 
straight  edge  of  the  protractor  is  placed 
upon  the  line  B  C,  so  that  the  center 
around  which  it  is  drawn  will  be  exactly 
at  the  intersection  of  the  lines,  or  point  of 
the  angle,  as  at  B ;  then  the  number  of  de- 
grees included  between  the  lines  on  the 
protractor  will  represent  the  quantity  or 
amount  of  the  angle.  From  this  it  will  be 
seen  that  the  amount  of  the  angle  does  not 
depend  upon  the  length  of  the  lines  which 
form  it,  nor  upon  the  magnitude  of  the 
circle  on  which  the  degrees  are  marked  by  which  it  is  measured,  but  simply  upon  tho 
width  of  the  opening  between  the  lines,  as  compared  with  the  whole  circumference 
&round  the  point  B.  A  circle  marked  off  into  degrees,  minutes,  and  seconds,  is  called  a 
graduated  circle. 

Circles  are  also  divided  into  Semicircles,  Quadrant*, 
Sextants,  Signs,  and  Arcs. 

A  Semicircle  is  the  half  of  a  cir- 
cle, or  186°. 

A  Quadrant  is  one  quarter  of  a 
circle,  or  90°. 

The  term  Quadrant  is  applied  to  a  nautical  instru- 
ment, of  the  form  of  a  quarter  of  a  circle,  which  is  much 
used  by  navigators  in  determining  the  altitude  or  appa- 
rent hight  of  the  sun,  moon,  and  stars. 

A  Sextant  is  the  sixth  part  of  a 
circle,  and  contains  60°. 

The  word  Sextant  also  denotes  an  instrument  similar  to  a  Quadrant,  and  is  used  for 
similar  purposes.  The  main  difference  is,  that  one  represents  60°,  and  the  other  90°, 
of  a  circle.  The  Octant,  or  eighth  part  of  a  circle,  is  also  used  for  similar  purposes. 

A  Sign  is  the  twelfth  part  of  a  circle,  or  30°. 
An  Arc  is  any  indefinite  portion  of  a  circle. 

The  word  Arc  is  from  the  Latin  a-rcus,  a  bow,  vault,  or  arch.  By  associating  the  woru 
cr«  with  arch,  the  student  may  always  remember  its  meaning. 

A  Chard  is  a  right  line,  joining  the 
extremities  of  an  arc. 

The  Chord  of  an  Arc  is  said  to  be  subtended  (from  #M&, 
under,  and  teno,  to  stretch),  because  it  seems  stretched  under 
the  arc  like  the  string  of  a  bow.  In  the  cut,  there  are  four 
arcs,  and  as  many  chords.  The  lower  arc  is  a  large  one, 
\\-liile  the  arc  and  chord,  A  C,  are  quite  small.  Still  each 
division  of  the  circle,  whether  great  or  small,  is  an  arc,  and 
the  line  joining  the  extremities  of  each  arc,  respectively,  iti  a 
clwrd. 


ABO  AND  CHORD. 


Describe.  A  graduated  circle  ?)  "What  larger  divisions  of  a  circle?  What  is 
a  semicircle  ?  A  quadrant?  (Note.)  A  sextant  ?  (Note.)  A  sign  ?  An 
urc  ?  fDerivatiou  of  term  ?)  Define  a  chord.  (Why  said  to  be  subtended  \) 


DEFINITIONS. 


23 


FOCI  OF  AN  ELLIPSE. 


ECCENTRICITY  OF  AN  ELLJPSK. 


An  Ellipse  is  an  oblong  figure 
like  an  oblique  view  of  a  circle, 
having  two  points  called  its  foci, 
around  which,  as  centers,  the  figure 
is  described. 

Foci  is  the  plural  of  focus. 

The  longer  diameter  of  an  ellipse 
is  called  its  Major  Axis,  and  the 
shorter  its  Minor  Axis. 

Axes  is  the  plural  of  axis.  The  longer  is  some- 
times called  the  Transverse,  and  the  shorter  the 
Conjugate,  Axis ;  but  major  and  minor  are  more  sim- 
ple and  perspicuous,  and  therefore  preferable. 

The  Eccentricity  of  an  ellipse 
is  the  distance  between  its  cen- 
ter and  either  focus. 

Eccentric — «B,  from,  and  centrum,  center. 
Hence  a  circle  that  varies  in  its  distance  from  the 
center  is  eccentric.  So,  also,  persons  who  depart 
from  the  usual  round  of  thought  and  custom  are 
called  eccentric  persons. 

19.  THE  TERRESTRIAL  SPHERE. 

The  Terrestrial  Sphere  is  the  earth  or  globe  we  in- 
habit. 

1.  Though  the  earth  is  not,  strictly  speaking,  a  sphere,  as  that  figure  is  denned  (14), 
but  rather  an  oblate  spheroid  (14),  still  it  is  usually  called  a  sphere  on  account  of  its 
near  approach  to  that  figure,  and  as  a  matter  of  convenience. 

2.  Terrestrial,  Latin  terrestris,  from  terra,  the  earth.    "There  are  also  celestial 
bodies,  and  bodies  terrestrial ;  but  the  glory  of  the  celestial  is  one,  and  the  glory  of  tho 
terrestrial  is  another." — 1  Cor.  xv.  40. 

The  Axis  of  the  earth  is  the  imaginary  line  about  which 
it  makes  its  daily  revolution. 

The  Poles  of  the  earth  are  the  extremities  of  her  axis 
where  they  cut  or  pass  through  the  earth's  surface. 

The  wire  upon  which  an  artificial  globe  turns  represents  tho  earth's  axis,  and  too 
extremities,  the  North  and  South  Polos. 

The  Equator  of  the  earth  is  an  imaginary  circle  drawn 
around  it,  from  east  to  west,  at  an  equal  distance  from 
the  poles,  and  dividing  it  into  two  equal  parts,  called 
Hemispheres. 

See  illustration,  page  18. 

An  ellipse?    Its  foci?    (Plural   and  singular?)    Major  and  minor  axes? 
(Singular  and  plural  ?)     Eccentricity  of  un  ellipse  ?     (Derivation  ?) 

19.  The  teriestrial  sphere?  (Is  the  earth  a  sphere?  Derivation  of  term 
terrestrial  ?)  A  xis  of  the  earth  ?  Polos  ?  Equator  ?  Latitude  ?  Purallcki  i 


ASTRONOMY. 


Latitude  upon  the  earth  is  distance  either  North  or 
South  of  the  Equator,  and  is  reckoned  each  way  toward 
the  Poles  in  Degrees,  Minutes,  and  Seconds. 

As  the  distance  from  the  Equator  to  the  Pole  cannot  be  more  than  a  quarter  of  a 
circle,  or  90°,  it  is  obvious  that  no  place  can  have  more  than  90°  of  latitude  ;  or,  iu 
other  words,  all  p!  ws  upon  the  earth's  surlace  must  be  between  the  Equator  and  90° 
of  latitude,  either  north  or  south. 


Parallels  of  Latitude  are  circles 
either  North  or  South  of  the  Equator, 
and  running  parallel  to  it. 

"We  may  imagine  any  conceivable  number  of  parallels 
between  the  Equator  and  the  Poles,  though  upon  most 
maps  and  globes  they  are  drawn  only  once  for  every  ten 


THE  TROPICS  AND  POLAB 
CIRCLE. 


The  Tropics  are  two  parallels  of 
latitude,  each  23°  28'  from  the 
Equator. 

The  Northern  is  called  the  Tropic 
of  Cancer,  and  the  Southern  the 
Tropic  of  Capricorn. 

1.  The  Tropical  Circles  are  shown  at  E  E  in  the  an- 
nexed figure. 

2.  The  sun  never  shines  perpendicularly  npon  any 
points  on  the  earth  further  from  the  Equator  than  the 

Tropics.  Between  these  he  seems  to  travel  regularly,  leaving  the  Southern  Tropic  on 
the  23(1  of  December,  crossing  the  Equator  northwa'rd  on  the  20th  of  March,  midiing 
the  Northern  Tropic  on  the  21st  of  June,  crossing  the  Equator  southward  on  the  23d  of 
September,  and  reaching  the  Southern  Tropic  again  on  the  23d  of  December.  In  this 
manner  he  seems  to  cross  and  recross  the  Equator,  and  vibrate  between  the  Tropics 
from  year  to  year.  The  cause  of  this  apparent  motion  of  the  sun  will  be  explained 
hereafter. 

The  Polar  Circles  are  two  parallels  of  latitude,  23°  28' 
from  the  Poles.  (See  F  F  in  the  last  cut.) 

The  Northern  is  called  the  Arctic,  and  the  Southern 
the  Antarctic,  Circle. 

The  Tropics  and  Polar  Circles  divide  the  globe  into 
five  principal  parts,  called  Zones,  namely,  one  Torrid, 
two  Temperate,  and  two  Frigid. 

A  zone  properly  signifies  a  girdle  ;  but  the  term  is  here  used  in  an  accommodnte.1 
sense,  as  only  three  of  these  five  divisions  at  all  resemble  a  girdle.  The  parts  cut  oflf 
by  the  polar  circles  are  mere  convex  segments  of  the  earth's  surface. 

The  tropics  ?     Names?     Polar  circles  ?     Names?    Zones?    Names?    (Aro 
fiere  in  reality  any  frigid  zones?)    Situation  of  the  several  zones?    Men  1- 
lan?  ?     Longitude  on  the  earth  ?     First  meridian  ?     (Kuropean  and  Ameri 
ouu  charts  and  globes  ?)    How  longitude  reckoned  ?    Its  greatest  o,.M.eut  * 


DEFINITIONS. 


25 


The  Torrid  Zone  is  situated  between 
the  Tropics  ;  the  Temperate,  between 
the  Tropics  and  the  Polar  Circles  ;  and 
the  Frigid,  between  the  Polar  Circles 
and  the  Poles. 

Meridians  are  imaginary  lines  drawn 
from  pole  to  pole  over  the  earth's  sur- 
face. 

Meridians  cross  the  Equator  at  right  angles  ;  and  the 
plane  of  any  two  Meridians  directly  opposite  each 
other  would  divide  the  eartli  into  Eastern  and  Western 
Hemispheres,  as  the  Equator  divides  it  into  Northern 


THS  FIVE  ZONES. 


and   Southern.    We  rnay  imagine  Meridians  to  pass 

ivable  point  upo 
face.    They  meet  at  the  Poles,  and  are  furthest  apart 


point  upon  the  earth's  sur- 


through every  concei 
face.    They  me 
at  the  Equator. 

Longitude  upon  the  earth  is  dis- 
tance either  East  or  "West  of  any 
given  meridian. 

A  degree  of  longitude  at  the  Equator  comprises  about  69£  miles,  but  is  less  and 
less  as  the  meridians  approach  the  Poles,  at  which  points  it  is  nothing.  A  degree  of 
latitude  is  about  69  £  miles  on  all  parts  of  the  globe. 

The  First  Meridian  is  that  from  which  the  reckoning 
of  Longitude  is  commenced. 

On  European  charts  and  globes,  longitude  is  usually  reckoned  from  the  Eoyal  Ob- 
servatory at  Greenwich,  near  London;  "but  in  this  country  it  is  often  reckoned  from 
the  Meridian  of  Washington.  It  would  be  better  for  science,  however,  if  all  nations 
reckoned  longitude  from  the  same  Meridian,  and  all  charts  and  globes  were  constructed 
accordingly. 

As    Longitude    is 

reckoned  both  East 
and  West,  the  great- 
est longitude  that 
any  place  can  have 
is  180°. 

20.  THE  CELESTIAL 
SPHERE. 

The  Celestial 
Sphere  is  the  appa- 
rent concave  sur- 
face of  the  hea- 
vens, surrounding 
the  earth  in  all  di- 
rections. 

The  relation  of  tho  Terrestrial  to  the  Oclestia!  Sphere  may  be  understood  by  the 
*.bove  diagram,  in  "which  the  stars  surround  the  earth  in  all  directions,  as  they  seem  to 
Ail  the  whole  celestial  vault. 

Q 


ASTRONOMY. 


IQTTATOE  Or  TUB   KZATESS,  OR  IQlTINOCTIAlfe 


The  Axis  of  the  Heavens  is  the  axis  of  the  earth  pro 
duced  or  extended  both  ways  to  the  concave  surface  oi 
the  heavens. 

The  Equator  of  the 
heavens,  or  Equinoc- 
tial, is  the  plane  of 
the  Earth's  equator 
extended  to  the  starry 
heavens. 

Declination  is  dis- 
tance either  north  or 
south  of  the  Equinoc- 
tial. 

Declination  is  to  the  heavens 
precisely  what  latitude  is  upon 
the  earth.  It  is  reckoned  from 
the  celestial  equator,  both  North 
and  South,  to  90°,  or  to  the  poles 
of  the  heavens.  Celestial  Lati- 
tude can  be  explained  better 
hereafter,  and  so  with  the  terms 
Moniptio,  Zodiac,  <fcc. 

Right  Ascension  is  distance  east  of  a  given  point,  and 
is  reckoned  on  the  Equinoctial  quite  around  the  heavens. 

In  one  respect,  Eight  Ascension  in  the  heavens  is  like  longitude  on  the  earth: 
they  are  both  reckoned  upon  the  equators  of  their  re»i>ective  spheres.  But  while 
longitude  is  reckoned  both  east  and  west  of  the  first  meridian,  and  can  only  amount  tt> 
1800,  Right  Ascension  is  reckoned  only  eastward,  and  consequently  may  amount  to 
860°,  or  the  whole  circle  of  the  heavens.  The  principal  difference  between  Eight  As- 
cension and  Celestial  Longitude  is>  that  the  former  is  reckoned  on  the  Equinoctial,  and 
the  latter  on  the  Ecliptic. 

The  /Sensible  Horizon  is  that 
circle  which  terminates  our  view, 
or  where  the  earth  and  sky  seem 
to  meet. 

The  Rational  Horizon  is  an 
imaginary  plane,  below  the  visible 
horizon,  and  parallel  to  it,  which, 
passing  through  the  earth's  cen- 
ter, divides  it  into  upper  and  lower  hemispheres. 

1.  These  hemispheres  are  distinguished  as  upper  and  lower  with  reference  te  the  ob» 
aervcr  only. 


SENSIBLE    -I- HORIZON 


20.  Celestial  sphere  ?    (Kelation  to  terrestrial  ?)    Axis  of  the  heavens  ? 
Equator  of  the  heavens  ?     Declination  ?      (How  illustrated   by   terrestrial 
latitude  ?     How  reckoned  ?    Its  limits  ?;    Eight  ascension  ?    (How  resemM« 
What  difference  ?}    Sensible  horizon  ?    Rational  I    Explain  bj 


DEFINITIONS. 


2.  The  sensible  horizon  is  half  the  diameter  of  the  earth,  or  about  4,000  miles  from 
the  rational ;  and  yet  so  distant  are  the  stars,  that  both  these  planes  seein  to  cut  the 
celestial  arch  at  the  same  point;  and  we  see  the  same  hemisphere  of  stars  above  th? 
sensible  horizon  of  any  place  that  we  should  if  the  upper  half  of  tLe  earth  wero  re- 
moved, and  we  stood  on  the  rational  horizon  of  that  place. 

Tlie  Poles  of  the  Horizon  are  two  opposite  points- 
one   directly   above,    and   the    other   directly   beneath , 
us.     The  first  is  called  the  Zenith,  and  the  latter  the" 
Nadir. 

The  points  Tip  and  Down,  East  and  West,  are  not 
positive  and  permanent  directions,  but  merely  rela- 
tive. 


TTP  AND  DOWN,  AND  EAST  AND 
WEST. 


1.  As  the  earth  is  a  sphere,  inhabited  on  all  sides, 
the  Zenith  point  is  merely  opposite  its  center,  and  the 
Nadir  toicard  its  center.    So  with  the  directions  Up 
and   Down:  one  is  from  the  center,  and  the  other 
toward  it ;  and  the  same  direction  which  is  up  to  one, 
is  down  to  another.    This  fact  should  not  merely  be 
acknowledged,  but  should  be  dwelt  upon  until  the  mind 
has  become  familiarized  to  the  conception  of  it,  and  di- 
vested, as  far  as  possible,  of  the  notion  of  an  absolute 
up  and  down  in  space.     We  should  remember  that  we 
are  bound  to  the  earth's  surface  by  attraction,  as  so 
many  needles  would  be  bound  to  the  surface  of  a  spher- 
ical loadstone. 

2.  East  and  West  also  are  not  absolute,  but  merely 
relative,  directions.    East  is  that  direction  in  which 
the  sun  appears  to  rise,  and  West  is  the  opposite  direc- 
tion ;  and  yet,  so  far  as  absolute  direction  is  concerned, 
what  is  East  to  one,  as  to  the  observer  at  A,  is  West  to 

B,  and  so  with  C  and  D.  And  as  the  earth  revolves  upon  its  axis  every  twenty-four 
hours,  it  is  obvious  that  East  and  West  upon  its  surface  must,  in  that  time,  change  to 
every  point  in  the  whole  circle  of  the  heavens.  The  same  is  true  of  the  Zenith  and 
Nadir,  or  of  up  and  down. 

Space,  in  Astronomy,  is  that  boundless  interval  or  void 
in  which  the  earth  and  the  heavenly  bodies  are  situated, 
and  extending  infinitely  beyond  them  all,  in  every  direc- 
tion. 

Space  has  no  limits— or,  in  other  words,  is  boundless,  or  infinite.  Suppose  six 
persons  were  to  start  from  as  many  different  points  upon  the  earth's  surface — as,  for 
instance,  one  from  each  polo,  and  one  from  each  of  the  positions  occupied  by  observers 
in  the  next  figure.  Let  them  ascend  or  diverge  from  the  earth  in  straight,  lines,  perpen- 
dicularly, to  its  surface,  and  though  they  were  to  proceed  onward,  separating  from 
each  other,  with  the  speed  of  lightning,  for  millions  of  ages,  none  of  these  celestial 
voyagers  would  find  an  end  to  space,  or  any  effectual  barrier  to  hinder  their  advance- 
ment. Should  they  chance  to  meet  another  world  in  the  line  of  their  flight,  it  would 
soon  be  passed,  like  a  ship  met  by  a  mariner  upon  the  ocean,  and  beyond  it  space 
Mould  still  invite  them  onward  to  explore  its  immeasurable  depths.  And  thus  they 
might  go  on  forerer,  without  changing  their  position  in  respect  to  the  center  or  lonn- 
ddt-iesot  immensity  :  for  as  eternity  has  no  beginning,  middle,  or  end,  so  space  is  with- 
out center  or  circumference — an  ethereal  ocean,  without  bottom  or  shore. 


diagram.  Poles  of  the  horizon  ?  Names?  Up  and  down — positive  or  rela- 
tive points?  (Illustrate  by  diagram;  also  east  and  west.)  Term  space  in 
astronomy?  (Has  it  any  limits  ?  Illustration.) 


28 


ASTRONOMY. 


THE  60LAR  SYSTEM. 


SOLAR  8YRTKM  ANT)   SIDEREAL  HEAVENS. 


21.  FIRST  GRAND 
DIVISIONS  OF  THE 
UNIVERSE. 

The  visibls  uni- 
verse may  be  con- 
sidered under  two 
grand  divisions — 
viz.,  the  SOLAR  SYS- 
TEM and  the  SIDE- 
REAL HEAVENS. 

The  Solar  System 
consists  of  the  sun 
and  all  the  planets 
and  comets  that  re- 
volve around  him. 

The  Sidereal  Hea- 
vens include  all 
those  bodies  that  lie 
around  and  beyond 
the  Solar  System, 
in  the  region  of  the 
Fixed  Stars. 

1.  The   word    Sidereal    is 
from  the  Latin  sideralis,  and 
signifies  pertaining    to    the 
shirs.    The  Sidereal  Heavens 
are,  therefore,  the  heavens  of 
the  fixed  stars. 

2.  The  relation  of  the  Solar 
System  to  the  Sidereal  Hea- 
vens is  shown  in  the  annexed 
cut,  where  the  sun    appears 
only  as  a  star,  at  a  distance 
from  all  others.and  surrounded 
by  his  own  retinue  of  worlds. 
The  Solar  System   is  drawn 
upon   a  small   scale,  and  the 
Sidereal    Heavens    are     seen 
wound  and  at  a  distance  from 
it  in  every  direction. 

In  considering  the  general  subject  of  Astronomy,  we 
shall  proceed  according  to  the  foregoing  classification, 
treating  first  of  the  SOLAR  SYSTEM,  and,  secondly,  of  the 
SIDEREAL  HEAVENS. 

21.  How  visible  universe  divided?  Define  each?  (Derivation  of  term 
"side-real  ?  Relation  of  solar  system  to  the  sidereal  heavens  ?  Illustrate  by 
drawing.)  Of  which  division  does  the  author  first  treat  ? 


PART  I. 

THE  SOLAR  SYSTEM 


CHAPTER  I. 

THE     PRIMARY     PLANETS.          .. 

22.  The  Solar  System  derives  its  name  from  the  Latin 
term  sol,  the  sun.     It  signifies,  therefore,  the  System  of 
the  Sun.     It  includes  that  great  luminary,  and  all  the 
planets  and  comets  that  revolve  around  him. 

23.  The  Sun  is  the  center  of  the  system,  around 
which  all  the  solar  bodies  revolve,  and  from  which  they 
receive  their  light  and  heat. 

24.  The  Planets  are  those  spherical  bodies  or  worlds 
that  revolve  statedly  around  the  sun,  and  receive  their 
light  and  heat  from  him. 

The  term  planet  signifies  a  wanderer,  and  was  applied  to  the  solar  bodies  because 
they  seemed  to  move  or  wander  about  among  the  stars. 

The  Orbit  of  a  planet  is  the  path  it  pursues  in  its  revo- 
lution around  the  sun. 

25.  The  planets  are  divided  into  Primary  and  Secon- 
dary planets. 

The  Primary  Planets  are  those  larger  bodies  of  the 
system  that  revolve  around  the  sun  only,  as  their  center 
of  motion. 

The  Secondary  Planets  are  a  smaller  class  of  bodies, 

22.  Of  what  does  Part  II.  treat  ?    What  meant  by  the  Solar  System  ?    In 
eludes  what  ? 

23.  What  is  the  sun  ? 

24.  Describe  the  planets.     (The  term  ?)    The  orbit  of  a  planet  ? 

25.  I  low  planets  divided?    Describe  each.     (What  other  names  for  secon- 
daries *) 


SO  ASTRONOMY. 


that  revolve  not  only  around  the  sun,  but  also  around  the 
primary  planets,  as  their  attendants,  or  moons. 

The  secondary  planets  are  also  called  Moons  or  Satellites.    A  satellite  is  a  follower  or 
attendant  upon  another. 


OF  THE  SOLAR   SYSTEM. 


In  this  cut,  the  sun  may  be  seen  in  the  center.  The  white  circles  are  the  Orbits 
of  the  primary  planets.  The  planets  may  be  seen  in  those  orbits  at  various  distances 
from  the  sun.  The  numerous  orbits  so  close  together  are  those  of  the  Asteroids.  The 
Bicindary  planets  may  be  seen  near  their  respective  primaries,  revolving  around  them, 
while  they  all  go  on  together  around  the  sun.  On  the  right  is  seen  a  Comet  plunging 
Into  the  system,  with  his  long  and  fiery  train.  His  orbit  is  seen  to  be  very  elliptical. 
All  these  bodies  are  opake,  the  sun  excepted.  Even  the  blazing  comet  shines  only  by 
reflection. 

26.  The  planets  are  again  divided  into  Interior  and 
Exterior  planets. 

The  Interior  Planets  are  those  whose  orbits  lie  within 
the  orbit  of  the  earth,  or  between  it  and  the  sun. 

26.  What  meant  by  interior  and  exterior  planets  ?  (Why  not  inferior  and 
•uperior  ?) 


PRIMARY    PLANETS.  31 


The  Exterior  Planets  are  those  whose  orbits  lie 
out  the  orbit  of  the  earth. 

Some  Astronomers  speak  of  these  two  classes  respectively  as  Inferior  and  Superior. 
The  reason  seems  to  be,  that  as  those  nearer  the  sun  than  the  earth  are  lower  than  she 
Is — that  re,  nearer  the  great  center  of  the  system — they  arc,  in  this  respect,  inferior 
to  her;  while,  on  the  other  hand,  tiiose  that  are  above,  or  beyond  her,  are  her  superiors. 
But  as  the  distinction  is  founded  upon,  and  is  intended  to  denote,  the  position  of  the 
planets  with  respect  to  ti»e  eartlfs  orbit,  it  is  obvious  that  interior  and  exterior  are  the 
more  appropriate  terms.  It  seems  hardly  allowable  to  call  the  Asteroids  superior  plan- 
ets, and  Mercury  and  Venus,  which  are  much  larger,  inferior. 

27.  Comets  are  a  singular  class  of  objects,  belonging 
to  the  solar  system,  distinguished  for  their  long  trains 
of  light,  their  various  shapes,  and  the  great  eccentricity 
of  their  orbits. 


NUMBER  AND  NAMES  OF  THE  PRIMARY  PLANETS. 

28.  The   principal   Primary   Planets   are   Mercury, 
Venus,   Earth,    Mars,   Jupiter,   Saturn,    Uranus,   and 
Neptune.      Five  of   these,  including  the  Earth,  were 
known  to  the  ancients ;  but  Uranus  and  Neptune  have 
both  been  discovered  during  the  last  hundred  years. 

Besides  the  eight  larger  planets,  there  are  now  known 
to  exist  eighty-live  small  planets,  called  Asteroids,  all 
revolving  between  the  orbits  of  Mars  and  Jupiter. 
Four  of  these,  namely,  Ceres,  Pallas,  Juno,  and  Vesta, 
have  been  known  to  exist  since  1807.  The  remaining 
eighty-one  have  all  been  discovered  since  184:5,  and 
most  of  them  between  1852  and  1865.  (For  a  complete 
list  of  the  Asteroids,  see  page  247.) 

The  terra  Asteroid  signifies  star-like,  and  is  applied  to  these  small  planets  because 
of  their  comparative  minuteness.  They  are  never  seen  except  through  telescopes, 
and  through  ordinary  instru meats  are  awt  always  readily  distinguished  from  the  fixed 
stars. 

29.  The  Primary  Planets  are  denoted  in  astronomi- 
cal works  by  certain  signs  or  symbols;   and  as  their 
names   are   derived    from    Mythology,   their    symbols 
usually  relate  to  the  imaginary  divinities  after  whom 
they  are  named. 

27.  What  are  comets  ?    How  distinguished  ? 

28.  Number  of  the  principal  planets  ?    How  long  known  ?    What  other 
planets  ?    How  long  known  ?    What  said  of  their  discovery  ?    Meaning 
of  tlwj  term  Asteroid  ? 

2C.  How  are  the  planets  designated  in  astronomical  works  ?  (Describe 
the  preceding  cut.  Where  the'  sun  ?  Primaries  ?  Secondaries  ?  Aste- 
roids? Orbits?  Couiet  and  orbit?  Which  self-luminous^  and  which 
opake.) 


ASTRONOMY. 


ROD  OF  MERCURT 


1.  In  the  preceding  cut,  the  planets  are  placed  at  their  respective  distances  from 
the  sun,  as  nearly  as  can  be  represented  in  so  small  a  drawing;    The  orbits  of  tho 
asteroids  are  represented  by  a  few  whito  circles  only,  located  between  the  orbits  of 
Mars  and  Jupiter. 

2.  The  mythological  history  and  symbols  of  a  few  of  the  planets  will  now  be  given 
as  samples  of  the  whole,  many  of  the  asteroids  not  having  any  signs  attached  to 
their  names  as  yet  in  astronomical  works. 

MYTHOLOGICAL  HISTORY  AND  SYMBOLS. 

30.  MERCURY  was  the  messenger  of  the 
gods,  and  the  patron  of  thieves  and  dishon- 
est persons.     His  symbol  denotes  his  cadw- 
ceus,  or  rod,  with  serpents  twined  around 

it  x«).* 

1.  Mercury  was  represented  as  very  eloquent,  and  skillful  in  in- 
terpreting and  explaining — as  the  god  of  rhetoricians  and  orators. 
Hence,  when  Paul  and  Barnabas  visited  Lystra,  addressed  the^peo- 
ple,  and  wrought  a  miracle,  they  said,  "  The  gods  have  corne  down 
to  us  in  the  likeness  of  men.    And  they  called  Barnabas  Jupiter, 
and  Paul  Mercurius,  because  he  icas  the  chief  speaker," 

2.  "The  caduceus  of  Mercury  was  a  sort  of  wand  or  scepter,  borne  by  Mercury  as  an 
ensign  of  quality  and  office.    On  medals,  it  is  a  symbol  of  good  conduct,  peace,  and 
prosperity.    The  rod  represents  power;  the  serpents,  wisdom,;  and  the  two  icing», 
diligence  and  activity.'" — ENCYCLOPAEDIA. 

8.  The  original  form  of  this  sign  may  be  understood  by  the  preceding  cut,  to  which  the 
present  astronomical  symbol  (  Q  )  bears  but  a  slight  resemblance. 

31.  YENUS  was  the  goddess  of  love 
and  beauty,  and  her  sign  is  an  ancient 
mirror  or  looking-glass  ( 9 ),  which  she  is 
represented  as  carrying  in  her  hand. 

Anciently,  mirrors  were  made  of  brass  or  silver,  highly  pol- 
fshed,  so  as  to  reflect  the  image  of  whatever  was  brought  before 
them.  Hence  it  is  said  in  the  Book  of  Exodus,  written  fifteen 
centuries  before  Christ,  that  Moses  "made  the  laver  of  brass, 
and  the  foot  of  it  of  brass,  of  the  looking-glasses  of  the  women," 
&c.  For  convenience,  the  ancient  mirrors  had  a  handle  at- 
tached, as  represented  in  the  cut,  which  very  much  resembles 
the  sign  of  the  planet. 

32.  THE  EARTH  (called  by  the  Greeks 
Ore,  and  by  the  Latins  Terra)  has  two  sym- 
bols— one  representing  a  sphere  and  its  equator  (0),  and 
the  other  (©)  the  four  quarters  of  the  globe. 

*  All  these  symbols  should  be  drawn  in  rotation  upon  the  Blackboard,  during  recita- 
tion, by  the  Teacher,  or  some  member  of  tho  class.  It  will  be  well,  therefore,  for  t?ie 
student  to  observe  each  sign  carefully,  that  he  may  be  prepared  to  draw  and  explain 
it,  if  called  upon. 

80.  Who  was  Mercury,  in  Mythology,  and  what  does  his  symbol  denote 
(How  was  he  represented  ?    What  Scriptural  allusion  ?    Pescribe  his  cadu- 
cous.    The  meaning  of  its  parts  ?) 


MIBROR  OF  VENUS. 


MYTHOLOGICAL   HISTORY   AND  SYMBOLS.  33 

33.  MAES  was  the  god  of  war,  and  his  sign 
(  $  )  represents  an  ancient  shield  or  buckler.   SPBAR  ATn>  SHIKI-D 

It  OF  MARA 

crossed  by  a  spear. 

Gunpowder  was  not  known  to  the  ancients,  consequently  they 
had  no  pistols,  muskets,  or  cannon.  They  fought  with  short  swords 
and  spears,  and  defended  themselves  with  the  shield,  carried  on 
the  left  arm.  A  shield  and  spear  were,  therefore,  very  appropriate 
emblems  of  war.  The  original  form  of  the  sign  of  Mars  is  pre- 
sented in  the  cut 

3±.  FLOKA  was  the  "  queen  of  all  the 
flowers,"  and  her  symbol  (5??)  is  a  flower. 
the  "Kose  of  England." 

35.  CLIO  was  one  of  the  Muses.     Her  sign 
star,  with  a  sprig  of  laurel  over  it. 

36.  VESTA  was  the  goddess  of  fire,  and  her  sign  (£)  is 
an  altar,  with  &fire  blazing  upon  it. 

37.  IRIS  was  the  beautiful  waiting-maid  of  Juno,  the 
queen  of  heaven.     Her  symbol  (&*)  is  composed  of  a 
semicircle,  representing  the  rainbow,  with  an  interior 
star,  and  a  base  line  for  the  horizon. 

"As  an  attendant  upon  Juno,"  says  Prof.  Hind,  "the  name  was  not  inappropriate  at 
the  time  of  discovery,  when  Juno  was  traversing  the  18th  hour  of  right  ascension,  and 
was  followed  by  Iris  in  the  19th." 

38.  METIS  was  the  first  wife  of  Jupiter,  and  the  god- 
dess of  prudence  and  sagacity.     Her  symbol  («,)  is  an 
eye  (denoting  wisdom)  and  a  star.  • 

39.  HEBE  presided  over  children  and  youth,  and  was 
cup-bearer  to  Jupiter.     Her  sign  (2)  is  a  cup. 

Hebe  was  celebrated  for  her  beauty,  but  happening  one  day  to  stumble  and  spill  the 
nectur,  as  she  was  serving  Jupiter,  she  was  turned  into  an  footer,  and  doomed  to  haruesa 
and  drive  the  peacocks  of  the  queen  of  heaven. 

40.  PARTHENOPE  wTas  one  of  the  three  Syrens —  a  sea 
nymph  of  rare  beauty.    They  were  all  admirable  singers; 
hence  a  lyre  (l)  is  her  appropriate  sign. 

1.  The  three  Syrens — Parthenope,  Ligeia,  and  Leucosia — were  represented  as  dwoll- 

81.  Venus  and  symbol  ?    (Ancient  mirrors  ?    Scripture  allusion  ?) 

32.  The  Earth — anci-ut  name  and  symbols  ? 

33.  Mars  and  symbol  i    (Ancient  mode  of  warfare  ?  > 

34.  Flora  and  sign  ? 

35.  Clio  and  symbol  ? 

86.  Vesta  and  her  symbol  ? 

87.  Iris  and  her  sign  ?    (Prof.  Hind's  remark  ?* 

88.  Metis  and  her  sign  ? 

89.  Hebe  and  her  sign  I    (Incident  mentioned  in  note  ?) 

40.  Parthenope  and  sign  f  (What  said  of  'Jia  Syrens  3  Of  the  appro 
priateness  of  the  name  ?) 

2* 


34:  ASTRONOMY. 


ing  upon  the  coast  of  Sicily,  and  luring  mariners  upon  the  rocks  of  destruction  by  their 
enchanting  songs.    Hence  whatever  tends  to  entice  or  seduce  to  ruin  is  often  called  a 


1  syren  soi 


As  this  planet  was  discovered  at  the  Naples  Observatory,  in  Italy,  it  was  quite  ap- 
f  ropriate  to  name  it  after  one  of  the  Syrens,  that  Mythology  located  on  the  coast  of  a 
neighboring  island. 

41.  EGERIA  was  the  counsellor  of  Kuma  Pompilius. 
Symbol  not  yet  agreed  upon  by  astronomers. 

42.  ASTRJEA  was  the  goddess  of  Justice,  and  her  sign 
(ill)  is  a  balance. 

Mythology  teaches  that  Justice  left  heaven,  during  the  golden  age,  to  reside  on  the 
earth ;  but  becoming  weary  with  the  iniquities  of  men,  she  returned  to  heaven,  and 
commenced  a  constellation  of  stars.  The  constellations  Virgo  and  Libra  in  the  zodiac 
are  representations  of  Astnea  and  her  golden  scales.  So  the  female  figure,  holding  a 
pair  of  scales,  in  the  coat  of  arms  of  several  of  the  United  States,  is  a  representation  of 
Astrtea,  and  denotes  Justice. 

43.  IRENE  was  one  of  the  Seasons.    The  planet  was  so 
named  by  Sir  John  Herschel,  in  honor  of  the  peace  pre- 
vailing in  Europe  at  the  time  of  its  discovery  (May, 
1851).     Its  symbol  (3^)  is  a  dove,  with  an  olive  branch  in 
her  mouth,  and  a  star  upon  her  head. 

44.  EUNOMIA  was  another  of  the  Seasons — a  sister  cf 
Irene.     (Symbol  not  ascertained.) 

45.  JUNO  was  the  reputed  queen  of  heaven,  and  her 
sign  (  $ )  is  an  ancient  mirror,  crowned  with  a  star — an 
emblem  of  beauty  and  power. 

46.  CERES  was  the  goddess  of  grain  and  harvests,  and 
her  sign  (?)  is  a  sickle. 

47.  PALLAS  (or  Minerva)  was  the  goddess  of  wisdom 
and  of  war.     Her  symbol  ( $  )  is  the  head  of  a  spear. 

1.  The  ancient  Palladium  was  an  image  of  Pallas,  preserved  in  the  castle  of  the  city 
of  Troy ;  for  while  the  castle  of  the  city  of  Minerva  was  building,  they  say  this  image 
fell  from  heaven  into  it,  before  it  was  covered  with  a  roof. — Tooke's  Pantheon. 

2.  To  a  similar  fable,  respecting  an  image  falling  from  heaven,  the   Town-Clerk  al- 
ludes, Acts  xix.  35: — "Ye  men  of  Ephesus,  what  man  is  there  that  knoweth  n)t  how 
that  the  city  of  Ephesus  is  a  worshiper  of  the  great  goddess  Diana,  and  of  the  imago 
which  fell  down  from  Jupiter  ?" 

41.  Egeria  and  her  symbol  ? 

42.  Astraea  and  sign  ?    (Mythological  legend  ?    Virgo  and  Libra  ?    Where 
else  found  ?) 

43.  Irene— by  whom  named,  and  why  ?    Symbol  ? 

44.  Eunomia  and  symbol  ? 

45.  Juno  and  symbol? 

46.  Ceres  and  her  symbol? 

47.  Pallas  and  her  symbol  ?     (Ancient  Palladvum  f     Keputed  origin  t 
Scrip4  iral  allusion  to  it  ?) 


MYTHOLOGICAL   HISTORY   AND   SYMBOLS. 


35 


SATURN,  OR  CHRONOS. 


£8.  EYGEIA  was  the  goddess  of  health,  and  the  daugh- 
ter of  Esculapius,  the  father  of  the  healing  art.  (Symbo) 
not  ascertained.) 

Our  TtT'^ern  word  Hygtian,  which  signifies  the  laws  of  health,  Is  derived  from  the 

goddess  Hyg  Utt. 

49.  JWiTER  was  the  reputed  father  of  the  gods — the 
king  of   heaven.     His  symbol  (^)  was  originally  the 
Greek  letter  £,  zeta,  (the  same  as  our  Z) — the  initial  of 
the  Greek  word  Zeus,  the  name  for  Jupiter. 

50.  SATURN — called  by  the 
Greeks      Chronos  —  presided 
over    time     and    chronology. 
His    sign    (*?)    represents    a 
scythe. 

1,  Saturn  was  represented  in  Mythology  as 
an  old  man,  with  wings,  bald  excepting  a  fore- 
lock, with  a  scytlie  in  one  hand,  and  an  hour- 
glass in  the  other.    The  same  figure  is  now 
used  to  represent  time. 

2.  Our   modern    word     chronology,    from 
chronoa,  time,  and  logos,  discourse,   signifies 
the  science  of  keeping  time,  dates,  &c, 

51.  URANUS  was  the  father 
of  Saturn,  and  presided  over 
astronomy.     The    symbol    of 
this  planet  (r#)  consists  of  the 

letter  II,  with  a  planet  suspended'  from  the  cross-bar,  in 
honor  of  Sir  William  Herschel,  its  discoverer. 

This  planet  is  popularly  known  by  the  name  of  Herschel,  hut  astronomers  now  almost 
universally  call  it  Uranus.  It  bears  this  name  in  the  British  Nautical  Alman-ao  for 
1 651,  with  the  full  consent  of  Sir  John  Herschel,  the  son  of  the  great  discoverer.  It  was 
first  called  Georgium  Sidus,  by  Dr.  Herschel,  iu  honor  of  his  royal  patron,  George  HI. 

52.  NEPTUNE  was  the  god  of  the  seas,  but  the  symbol 
of  the  planet  (IP)  is  composed  of  an  L  and  a  Y  united, 
with  a  planet  suspended  from  the  hair-line  of  the  V,  in 
honor  of  Le  Yerrier,  its  discoverer. 

This  planet  was  first  called  Le  Verrier,  but  is  more  generally  known  by  the  name  of 
Neptune. 

53.  The  MOON  was  called  Luna  by  the  Romans,  and 

48.  Hy^eia  and  symbol  ?    (Term  Tiygeian  ?) 

49.  Jupiter  and  his  symbol  ? 

50.  Saturn  ?    Greek  name  ?    Symbol  ?    (How  represented  in  Mythology 
Word  chi'&rwlogy  f) 

51.  Uranus  and  symbol  ?    (What  other  names,  and  why?) 

52.  Neptune  and  his  symbol  ?    (Former  name  ?) 


36  ASTRONOMY . 


Selene  by  the  Greeks.  She  is  known  by  various  sym- 
bols, according  as  she  is  new,  half-grown,  or  fall, 
thus:  •,  ©,  O. 

.  1.  From  Luna,  we  have  OUT  modern  terms  lunar  and  Itvnctcy ;  the  former  of  which 
signifies  pertaining  to  the  moon,  and  the  'alter  a  disease  anciently  supposed  to  be  caused 
by  the  moon. 

2.  Selene,  in  Mythology,  was  the  daughter  of  Helio*,  the  Son.  Our  English  word 
selenography — a  description  of  the  moon's  surface — is  from  Selene,  her  ancient  tame, 
aud  grapho,  to  describe. 

54.  The  SUN — called  Sol  by  the  Romans,  and  Helios 
by  the  Greeks — is  represented  by  a  shield  or  buckler, 
thus  :  0,  ©,  ©.     As  the  large  and  polished  bucklers  of 
the  ancients  dazzled  the  eyes  of  their  enemies,  this  in- 
strument was  selected  as  an  appropriate  emblem  of  the 
sun. 

DISTANCES  OF  THE  PLANETS. 

55.  The  orbits  of  all  the  planets  being  more  or  less 
elliptic,  they  must  vary  in  their  distances  from  the  sun 
in  proportion  to  the  ellipticity  of  their  respective  orbits, 
and  their  position  in  their  orbits.     The  following  table 
exhibits  the  mean  or  average  distances  of  the  several 
planets  from  the  sun,  commencing  with  Mercury  and 
proceeding  outward. 

Mercury    ...         37  millions,  or      37,890,000 

Yenus  ....         69  "  68,770,000 

Earth    .     .     .     .  T      95  "  95,298,260 

Mars     ....        145  "  145,205,000 

The  Asteroids,  from  210  "  to     300,000,000 

Jupiter      ...        496  "  or     495,817,000 

Saturn       ....       909  "  909,028,000 

Uranus      .     .     .     1,828  "  1,828,071,000 

Neptune    .     .     .     2,862  «  2,862,457,000 

1.  The  first  column  of  round  numbers  only  should  be  committed  to  memory  by  the 
student  These  should  be  well  fixed  in  the  mind,  as  it  will  greatly  facilitate  the  pro- 
gress of  the  student  hereafter.  The  family  of  Asteroids  being  less  important,  their 
distances  need  not  be  learned  in  detail. 

It  is  impossible  for  the  human  mind  to  form  any  adequate  conception  of  the  dis- 
tance represented  by  the  phrase  ua  million  of  miles."  It  is  only  by  conceiving  aright 
in  regard  to  short  distances,  and  then  using  illustrations  and  instituting  comparisons, 
that  we  can  form  any  distinct  idea  of  these  really  inconceivable  spaces. 


53.  The  Moon — Latin  and  Greek  names  ?    Symbols  ?    (Words  lunar  anr 
lunacy  ?    Who  was  Selene  in  Mythology  ?    Selenography  ?    Derivation  ?) 

54.  The  Sun — Latin  and  Greek  names?     Symbol,  and  why? 

55.  Tiehearse,  in  round  numbers,  the  distances  of  the  planets  from  the 
(Substance  of  note  1st  I    Object  of  note  2d  ?    Note  3d  <    Note  4th  ?) 


DISTANCES  OF  THE  PLANETS.  37 


2.  The  comparative  distances  of  the  planets  are  represented  in  the  cut,  page  15  and 
*Iso  in  the  following : 


OF    THE    PLANETS. 


/+  I  I 

3.  To  assist  his  conception  of  these  vast  distances,  the  student  may  imagine  a  rail- 
road laid  down  from  the  sun  to  the  orbit  of  Neptune.    Now  if  the  train  proceed  from 
the  sun  at  the  rate  of  thirty  miles  an  hour,  without  intermission,  it  will  reach  Mercury 
in  152  years;  the  Earth  in  361  years:  Jupiter  in  1.884  years;  Saturn  in  3.493  years,- 

Uranux  in  6,938:  and  -Neptune  in  10.800  years  i    Such  a  journey  would  be  equal  to 
riding  900,000  times  across  the  continent,  from  Boston  to  Oregon ! 

4.  It   is  now  about  5.870  years  since  the  creation  of  man.     Had  a  train  of  cars 
started  from  the  sun  at  that  time  toward  the  orbit  of  Neptune,  and  traveled  day  and 
night  ever  since,  it  would  still  be  284  millions  of  miles  within  the  orbit  of  Uranus — 
about  where  the  head  of  the  locomotive  stands,  as  shown  in  the  cut!    To  reach  even 
that  planet  would  require  over  1.000  years  longer;  and  to  arrive  at  Neptune,  nearly 
6,000  years  to  come  !    Such  is  the  vast  area  embraced  within  the  orbits  of  the  planets. 
and  the  spaces  over  which  the  sunlight  travels,  to  warm  and  enlighten  its  attendant 
worlds. 

56.  The  apparent  magnitude  of  the  heavenly  bodies 
depends  much  upon  the  distance  from  which  they  are 
viewed ;  the  magnitude  increasing  as  the  distance  is 
diminished,  and  diminishing  as  the  distance  is  increased. 

NEAR    AND    REMOTE    VIEWS    OF    THE    SAME    OBJECT. 


Let  A  represent  the  position  of  an  observer  upon  the  earth,  to  whom  the  sun  appears 
32'.  or  about  half  n  degree  in  diameter.  Now  it  is  obvious  that  if  the  observer  advance 
to  B  (half  way),  the  object  will  fill  an  angle  in  his  eye  ttcice  as  large  a?  it  filled  when 
viewed  from  A.  Again:  if  he  recede  from  A  to  C,  the  object  will  appear  but  half  as 
large.  Hence  the  rule,  that  the  apparent  magnitude  is  increased  ^s  the  distance  is 
diminished,  and  diminished  as  the  distance  is  increased. 

57.  Could  a  beholder  leave  the  earth,  and,  descending 
toward  the  sun,  station  himself  upon  Mercury,  he  would 
find  the  apparent  magnitude  of  the  sun  vastly  increased. 
Should  he  then  return,  and  pass  outward  to  Mars  or 
Jupiter,  he  would  observe  a  corresponding  diminution  in 
the  sun's  magnitude,  in  proportion  as  the  distance  was 
increased.  Hence  the  apparent  magnitude  must  vary 

56.  How  apparent  magnitudes  of  heavenly  bodies  modified  1  (Illustrate 
hy  diagram.) 

*57.  Suppose  a  person  to  go  to  Mercury — what  effect  upon  apparent  size  of 
the  sun  1 


38  ASTRONO^IY. 


exceedingly,  as  viewed  from  different  points  in  the  solar 
system. 

THE    SUN,   AS    SEEN    FROM    THE    DIFFERENT    PLANETS. 

From 

N.    H.    S.  Jupiter.    Mars. 


The  above  cut  represents  the  relative  apparent  magni- 
tude of  the  sun,  as  seen  from  the  different  planets.  In 
angular  measurements,  its  diameter  would  be  as  follows  : 


From  Mercury     .     82^' 
"     Yemis   .     .     44^' 
"     Earth     .     .    32' 
"     Mars      .     .     21' 

The  Asteroids,  say  12' 


From  Jupiter      .     .     6' 
"      Saturn  ...     3^ 
"      Uranus      .     .     If 
"     Neptune   .     .  50" 


Let  us  continue  our  imaginary  journey  outward,  be- 
yond Neptune,  toward  the  fixed  stars,  and  in  a  short 
time  the  glorious  sun,  so  resplendent  and  dazzling  to  our 
view,  will  appear  only  as  a  sparkling  star  •  and  the  fixed 
stars  will  expand  to  view  as  we  approach  them,  till  they 
assume  all  the  magnitude  and  splendor  of  the  sun  him- 
self. 

LIGHT   AND   HEAT    OF   THE   PLANETS. 

58.  As  the  distances  of  the  planets,  respectively,  affect 
the  apparent  magnitude  of  the  sun,  as  viewed  from  their 
surfaces,  so  it  must  affect  the  relative  amount  of  light 
and  heat  which  they  respectively  receive  from  this  great 
luminary. 

59.  The  amount  of  light  and  heat  received  from  the 
sun,  by  the  several  planets,  is  in  inverse  proportion  to 
the  square  of  their  respective  distances. 

58.  What  effect  lias  the  distances  of  the  planets  from  the  sun,  respectively 
upon  their  relative  "tght  and  heat  ? 
69.  What  rule  governs  the  diffusion  of  light  ?    (Illustrate  ly  a  diagram.) 


LIGHT  AND   HEAT   OF   THE   PLANETS. 


39 


PHILOSOPHY    OF   THE    DIFFUSION    OF    LIGHT. 


1.  Here  the  light  is  *e«n  passing  in  right  lines,  from  the  sun  on  the  left  towarJ  the 
several  planets  on  therijht.     It  is  rfiso  shown  that  the  surfaces  A,  B,  and  C  receive  "<mal 
quantities  of  light,  though  Bis  four  times,  and  C  nine  times,  as  large  as  A  ;  and  as  the 
light  falling  upon  A  is  spreid  over  four  times  as  much  surface  at  B,  and  nine  times  as 
much  at  C,  it  follows  that  it  is  only  one-ninth  as  intense  at  C,  and  oive-fourth  at  B,  as  it 
ts  at  A.    Hence  the  rule,  that  the  light  and  heatoftheplanetaar^imversely,  as  the 
square*  of  their  respective  dirtonces. 

2.  The  student  may  not  exactly  understand  this  last  statement.    The  square  of  any 
number  is  its  product,  when  multiplied  by  itself.    Now  suppose  we  call  the  distances 
A,  B,  and  C  1,  2,  ami  3  miles.    Thtn  the  square  of  1  is  1  ;  the  square  of  2  is  4;  and  the 
square  of  3  is  9.    The  light  and  heat,  then,  would  be  in  inverse  proportion  at  these 
three  points,  as  1,  4,  and  9  ;  that  is,  four  times  less  atB  than  at  A,  and  nine  times  less  at 
C.    These  amounts  we  should  state  i£  1,  4,  and  i 

60.  The  intensity  of  light  and  heat  received  upon  the 
several  planets  varies,  according  to  their  respective  dis- 
tances, from  6J  times  as  mach  as  our  globe  to  g^th  part 
as  much. 


1.  The  comparative  light  and  heat  a   the  planets—  the  earth  being  1—  is  as  fol- 
lows: 


Jupiter . , 
Saturn . . . 
Uranus . . 
lleptune. 


Mercury 6  j 

Venus 2 

The  Earth 1 

Mars i 

The  Asteroids 1 

2.  From  this  table  it  appears  that  Mercury  Ji  us  6£  times  as  much  light  and  heat  as  our 
globe,  Uranus  only  ^--g,  and  Neptune  only  -jj^th  part  as  much.  Now  if  the  average 
temperature  of  the  earth  is  50  degrees,  the  average  temperature  of  Mercury  would  be 
825  degrees;  arid  as  water  boils  at  212,  the  temperature  of  Mercury  must  be  113  degrees 
above  that  of  boiling  water.  Venus  would  have  an  average  temperature  of  100  degrees, 
which  would  be  twice  that  of  the  earth.  On  the  other  hand,  Jupiter,  Saturn,  Uranus, 
and  Neptune,  seem  doomed  to  the  rigors  of  porpetual  winter.  And  what  conception 
can  we  form  of  a  region  900  times  as  cold  as  our  globe  I  Surely, 

"  "Who  there  inhabit  must  have  other  powers, 
Juices,  and  veins,  and  sense,  and  life,  than  ours; 
One  moment's  cold,  like  theirs,  would  pierce  the  bone, 
Freeze  the  heart's  blood,  and  turn  us  all  to  stone !" 

8.  It  is  not  certain,  however,  that  the  heat  is  proportionate  to  the  light  received  by 


60.  Between  what  limits  does  the  light  and  heat  of  the  several  planets 
vary  ?  (What  would  that  be  for  Mercury  ?  For  Venus  1  How  with  the  ex 
terior  planets  ?  Poetry  1  Is  it  certain  that  the  heat  of  the  planets  is  in  exact 
proportion  to  the  light  they  respectively  receive  1  Why  not  2) 


40  ASTRONOMY 


the  respective  planets,  as  various  local  causes  may  conspire  to  modify  clthei  extreme  of 
the  high  or  low  temperatures.  For  instance,  Mercury  may  liave  an  atmosphere  that  ar- 
rests the  light,  and  screens  the  body  of  the  planet  from  the  insupportable  rays  of  the 
«un ;  while  the  atmospheres  of  Saturn,  Ilerschel,  &c.,  may  act  as  a  refrsk-ting  medium  to 
gather  the  light  for  a  great  distance  around  them,  and  concentrate  it  upon  their  other- 
wise cold  and  dark  bosoms. 


MAGNITUDE   OF   THE   PLANETS. 

61.  The  planets  vary  as  much  in  their  respective  mag- 
nitudes^ as  in  their  distances.  Their  several  diameters, 
so  far  as  known,  are  as  follows  : 


Mercury     .     .     .  2,950 

Venus    ....  7,900 

Earth    ....  7,912 

Mars      ....  4,500 

Flora     .     .     .     .  — 

Clio — 

Vesta    ....  295 

Iris — 

Metis     ....—- 

Hebe     .     .     .     .  — 

Parthenope    .     .  — 

Egeria  ....  — 


Astrsea  ....  — 

Irene — 

Eunomia    ...  — 

Juno     ....  1,400 

Ceres    ....  163 

Pallas  ....  770 

Hygeia.     ...  — 

Jupiter.     .     .     .  88,780 

Saturn  ....  73,484 

Uranus.     .     .     .  36,000 

Neptune    .     .     .  35,000 


1.  The  asteroids  are  so  small  and  so  remote,  that  measurements  of  their  exact  diam- 
eters are  obtained  with  great  difficulty ;  hence  the  numerous  blanks  in  the  above  table. 
And  even  when  diameters  are  given,  they  are  somewhat  doubtful. 

2.  In  the  case  of  the  other  planets,  we  have  given  their  mean  or  average  diameters, 
according  to  the  best  authorities.    As  most  of  them  are  more  or  less  oblate,  their  polar 
diameters  are  less,  and  their  equatorial  more,  than  the  amount  given  in  the  table. 

62.  The  magnitude  of  the  principal  planets,  as  com- 
pared with  the  earth,  is  as  follows  : — Mercury,  y1^  as  large ; 
Venus,  T9o  ;  Jupiter,  1,400  times  as  large ;  featurn,  1,000 
times  ,  Uranus,  90  times ;  and  Neptune,  60. 

1.  The  magnitudes  of  spherical  bodies  are  to  each  other  as  the  cubes  of  their  diam- 
eters. Thus,  79l2x7912x7912=:49o,2S9,174,428,  the  cube  of  the  earth's  diameter;  and 
2950x2950x2950=25,672,375,000,  the  cube  of  the  diameter  of  Mercury.  Divide  the 
former  by  the  latter,  and  we  have  19  and  a  fraction  as  the  number  of  times  the  bulk  of 
Mercury  is  contained  in  the  earth. 


f>\.  State  the  diameters  of  the  several  planets  ?  (Why  blanks  in  the 
table  ?  What  diameters  are  given — polar,  equatorial,  or  neither  \) 

*>2.  Give  the  magnitude  of  the  principal  planets,  as  compared  with  the 
.f,arth.  (How  ascertain  relative  magnitudes  ?  How  possible  that  a  mere  star 
can  be  such  an  iinaieiise  wond  !) 


DENSITY. 


41 


COMPARATIVE    MAGNITUDE    OF    THE    SUN    AND    PLANETS. 


2.  It  may  seem  almost  incredible  that  what  appear  only  as  small  stars  in  tho  heavens 
should  be  larger  than  the  mighty  globe  upon  which  we  dwell.  But  when  we  consider 
their  immense  distance,  and  the  effect  this  must  have  upon  their  apparent  magnitude, 
as  illustrated  at  55,  it  is  evident  that  the  planets  could  not  be  seen  at  all  were  they  not 
very  large  bodies.  The  above  cut  will  give  some  idea  of  the  magnitude  of  the  several 
planets,  as  compared  with  each  other,  and  also  with  the  sun. 

63.  The  Sun  is  1,400,000  times  as  large  as  our  globe, 
and  500  times  as  large  as  all  the  other  bodies  of  the  solar 
system  put  together.  It  would  take  one  hundred  and 
twelve  such  worlds  as  our  earth,  if  laid  side  by  side,  to 
reach  across  his  vast  diameter. 


DENSITY. 

64.  The  planets  differ  greatly  in  their  density,  or  in  tho 
compactness  of  the  substances  of  which  they  are  com- 
posed. Mercury  is  about  three  times  as  dense  as  oui 
globe,  or  equal  to  lead.  Venus  and  Mars  are  about  the 
same  as  the  earth  ;  while  Jupiter  and  Uranus  are  only  -Jtli 
as  dense,  or  about  equal  to  water.  Saturn  has  only  j^th 
the  density  of  our  globe,  answering  pretty  nearly  to  cork-. 

63.  State  the  magnitude  of  the  sun  as  compared  with  the  earth.    With  the 
rest  of  the  system.    Illustration  ? 

64.  What  meant  by  density  f    Do  the  planets  differ  in  this  respect  ?    State 
and  ill-ostrate.    (How  masses  of  planets  ascertained  ?    How  with  M.ercury  ?) 


42  ASTKONOMY. 


The  masses  of  the  planets  are  determined  by  the  revolution  of  their  respective  satel- 
lites :  but  as  Mercury  has  no  satellite,  the  determination  of  his  mass  and  density  be- 
comes a  very  difficult  and  uncertain  matter.  "  But  it  fortunately  happens,"  says  Prof 
Hind,  "that  we  have  a  curious  method  of  approximating  to  this  element,  viz..  by  the 
perturbations  produced  by  the  planet  in  the  movements  of  a  comet  known  as  Encke's, 
which  revolves  around  the  sun  in  little  more  than  three  years,  and  occasionally  ap- 
proaches very  near  Mercury,  &c.  From  computations  based  upon  these  perturbations, 
I'rof.  II.  concludes  that  Mercury  is  only  about  y^-  more  dense  than  our  globe — a  result 
widely  different  from  that  arrived  at  by  his  predecessors. 

GRAVITATION. 

65.  Attraction,   or    Gravitation,  is   the  tendency  of 
bodies  toward  each  other.     It  is  that  tendency  which 
causes  bodies  raised  from  the  earth,  and  left  without  sup- 
port, to  fall  to  its  surface.  ATTRACTION  or  TH,  EJLRTH 

All  substances  fall  toward  th-s  earth's 
center  from  every  part  of  the  globe,  as  a 
spherical  loadstone  would  attract  parti- 
cles of  steel  to  its  surface  in  every  di- 
rection. Hence  when  these  four  men, 
etandingon  different  fides  of  the  globe, 
drop  each  a  stone,  tney  all  fall  toward 
the  same  point,  because  the  earth  at- 
tracts them  all  to  herself 

66.  Gravitation  is  what 
constitutes  the  weight  of 
bodies,  and  depends  upon 
the  quantity  of  matter  in 
the  bodies  attracting,  and 
their  distances  from  each 
other. 

The  reason  why  a  cubic  for,',,  of  cork  weighs  much  less  than  the  same  bulk  of  lead,  is, 
that  being  less  deusc,  ie  contains  much  less  matter  to  be  attracted. 

67.  From  the  above  law  of  attraction,  it  follows  that 
large  bodies  attract  much  more  strongly  than  small  ones, 
provided  their  densities  are  equal,  and  their  distances  the 
same ;  and  as  the  force  of  attraction  constitutes  the  weight 
of  a  body,  it  follows  that  a  body  weighing  a  given  num- 
ber of  pounds  on  the  earth,  would  weigh  much  more  on 
Jupiter  or  Saturn,  and  much  less  on  Mercury  or  the 
Asteroids. 

65.  Define  gravitation.    (What  illustration  given  ?) 

66.  What  relation  lias  gravitation  to  weight?    Upon  what  does  it  depend 
for  its  degree  of  force  \    (Why  is  a  cubic  foot  of  cork  lighter  than  the  same 
bulk  of  lead?) 

67.  What  effect  have  the  balk  and  density  of  the  planets  upon  the  tceigJit 
of  bodies  on  their  surfaces?     (State  comparative  weights.      IHustration? 
Why  not  attractive  force  or  weight  in  exact  proportion  to  bulk  *    How  nm*t 
i/xlie»  be  weighed  to  ascertain  difference,  aud  why  ?) 


GRAVITATION.  43 


1   The  following  table  shows  the  relative  attractive  force  of  the  sun  and  planets.    A 
body  weighing  one  pound  on  the  earth,  would  weigh, 

».  oz.  |  lb.  on. 

On  Mercury 1    lj  j  On  Saturn 1 


Venus 0  15 

"  Mars 0    8 

"  Jupiter 2    8 


Uranus 0  12£ 

"  Neptune unknown. 

"  The  Sun 28    5* 


2.  A  person  weighing  150  ibs.  on  the  earth  would  consequently  weigh  but  74  ibs.  upon 
Mars ;  while  upon  Jupiter,  his  weight  would  be  375  Ibs. ;  and  upon  the  sun,  4,250  Ibs. 1 
The  attractive  force  of  the  Asteroids  is  so  slight,  that,  if  a  man  of  ordinary  muscular 
strength  were  transported  to  one  of  them,  he  might  probably  lift  a  hogshead  of  lead 
from  its  surface  without  difficulty. 

S.  But  the  learner  will  notice  that  the  attractive  force,  as  shown  in  the  above  table,  is 
not  in  strict  proportion  to  the  bulk  of  the  planets  respectively.  This  difference  will  be 
accounted  for  by  considering  the  difference  in  their  density  (64).  From  the  principles 
there  laid  down,  it  will  be  seen  at  once,  that  though  one  planet  be  as  large  again  as  another, 
still,  if  it  were  but  half  as  dense,  it  would  contain  no  more  matter  than  the  smaller  one, 
and  their  attractive  force  would  be  equal.  If  Jupiter,  for  instance,  were  as  dense  as  the 
earth,  his  attractive  force  would  be  four  times  what  it  now  is;  and  if  the  density  of  all 
the  solar  bodies  were  precisely  the  same,  their  attractive  force,  or  the  weight  of  bo'dies  m. 
their  surfaces,  would  be  in  exact  proportion  to  their  bulk. 

4.  It  must  be  remembered,  however,  that  if  a  body  were  actually  weighed  upon  the 
surface  of  each  planet,  by  scales,  it  would  weigh  the  same  on  all,  because  the  force  of 
attraction  upon  the  w.iyhts  would  be  just  equal  to  that  of  the  body  to  be  weighed, 
whether  it  were  more  or  less.  With  a  steelyard  it  would  be  the  same.  A  spring"  and 
hook,  therefore,  is  the  only  instrument  with  which  we  could  weigh  objects  accurately  on 
the  different  planets. 

68.  If  the  earth  were  only  one-half  as  dense  as  she  now 
is,  it  would  reduce  the  weight  of  bodies  at  her  surface 
one-half.     So  if  a  body  were  taken  from  the  earth's  sur- 
face half  way  down  to  her  center,  the  weight  would  be 
reduced  one-half.     At  her  center  it  would  be  nothing, 
because  the  attractive  force  would  be  the  same  in  all 
directions. 

In  this  cut,  the  diameter  of  the  earth  is  divided  into  four 
equal  parts — C.  D,  E,  and  F.  At  A,  the  whole  attraction 
amounts  to  four  pounds.  When  the  stone  reaches  B,  the 
part  C  attracts  as  strongly  upward  as  D  does  downward,  and 
their  forces  balance  each  other.  Then  as  C  and  D  mutually 
neutralize  each  other,  we  have  only  the  parts  E  and  F,  or  one- 
half  the  globe,  to  attract  the  stone  downward;  consequently 
the  attractive  force  would  be  only  half  as  great  at  B  as  at  A, 
and  the  stone  would  weigh  only  two  pounds. 

69.  The  force  with  which  bodies  gravi- 
tate toward  each  other  is  in  direct  pro- 
portion to  their  respective  masses,  and  in  inverse  propor- 
tion to  the  squares  of  their  distances. 

A  man  carried  upward  in  a  balloon  weishs  less  and  less  as  his  distance  from  the  earth 
is  increased.  The  same  law  holds  good  in  regard  to  the  planetary  worlds.  The  nearei 
a  planet  is  te  the  sun,  or  to  any  other  body,  the  stronger  the  mutual  gravitation. 

68.  How  effect  weight  of  bodies  on  the  earth  to  reduce  her  density  one- 
naif?  How  to  take  down  half  way  to  center  2  Quite  to  center  ?  (Illustrate 
by  diagram.) 

G'.>.  Give  the  exact  law  of  gravitation  ?  (What  said  of  a  man  ascending  m 
a  balloon  ?  Of  more  distant  planets  ?) 


44  ASTRONOMY. 


69.  This  great  law  was  disco vered  by  Sir  Isaac  New- 
ton, in.  1666.  He  was  then  only  twenty-four  years  of 
age. 

The  inquiry  which  led  to  the  discovery  is  said  to  have  been  suggested  to  the  mind  of 
this  youthful  philosopher  by  seeing  an  apple  fall  from  the  limb  of  a  tree.  "  What  dre* 
these  two  globes  (the  apple  and  the  earth)  together  ?" 

PERIODIC   REVOLUTIONS    OF   THE    PLANETS. 

TO.  The  planets  all  revolve  around  the  sun  from  west 
to  east,  or  toward  that  part  of  the  heavens  in  which  the 
sun  appears  to  rise. 

To  assist  his  conception  of  the  direction  in  which  the  planets  revolve,  the  student 
may  suppose  that  if  the  earth  was  in  her  orbit  beyond  the  sun,  at  12  o'clock,  she  would 
go  what  we  should  call  eastward,  which  would  be  the  same  direction  that  we  should 
call  westward  on  the  earth,  at  the  same  time ;  as  bodies  revolving  in  a  circle  move  in 
opposite  directions  on  opposite  sides  of  the  circle. 

71.  The  passage  of  a  planet  from  any  particular  point 
in  its  orbit,  around  to  the  same  point  again,  is  called  its 
periodic  revolution;  and  the  time  occupied  in  making 
such  revolution  is  called  its  period,  or  periodic  time 
The  periodic  times  of  the  principal  planets  are  as  fol 
lows : 


Tears.  Days. 


Mercury   ...  0  88 

Yenus       ...  0  225 

Earth  ....  1  -~ 

Mars  1  322 


Tears.     Davs. 


Jupiter   ...     11  317 

Saturn    ...     29  175 

Uranus   .     .    ^  84  27 

Neptune      .    T164:  226 


72.  The  periodic  times  of  the  Asteroids  vary  from 
1,200  to  2,000  days,  the  average  being  about  1,600 
days,  or  four  and  a-half  years.  This  resemblance  in 
the  time  of  their  revolutions  is  due  to  the  fact  that  they 
vary  but  slightly  in  their  distance  from  the  sun,  a  cir- 
cumstance which  governs  the  time  of  the  revolution  of 
all  the  planets. 

69.  When  and  by  whom  were  the  Laws  of  Gravitation  discovered  ?  How 
old  ?  (What  led  to  this  discovery  ?) 

10.  In  what  direction  do  the  planets  revolve  in  their  orhits  ?  (Give  illus- 
tration.) 

71 .  What  meant  by  the  periodic  revolution  of  a  planet  ?    Its  period  or  peri- 
odi.c  tim-f  ?    Give  the  periods  of  the  principal  planets. 

72.  Periodic  times  of  the  Asteroids  ?     Cause  of  agreement  ?    (What 
constitutes  the  year  of  a  planet?    Compare  years.) 


CENTRIPETAL  AND  CENTRIFUGAL  FORCES.  45 


HOURLY  MOTION  OF  THE  PLANETS  IN  THEIR  ORBITS. 

73.  The  velocity  with  which  the  planets  fly  through 
space,  in  performing  their  periodical  journeys  around  the 
sun,  varies  from  11,000  to  110,000  miles  an  hour.  The 
hourly  motion  of  the  earth  amounts  to  68,000  miles  ! 

1.  The  hourly  motion  of  the  planets  is,  approximately,  as  follows: 


Miles 


Mcrcnry 110,000 

Venus 75,000 

Earth 68,000 

Mars 55,000 


Miles. 


Jupiter 30,000 

Saturn 22,000 

Uranus 15,000 

Neptune 11,000 


Here,  instead  of  finding  the  swiftest  planets  performing  the  longest  periodic  journeys, 
this  order  is  reversed,  and  they  are  found  revolving  in  the  smallest  orbits.  The  nearer  a 
planet  is  to  the  sun,  the  more  rapid  its  motion,  and  the  shorter  its  periodic  time.  The 
reasons  for  this  difference  in  the  velocities  and  periodic  times  of  the  planets,  will  appear 
in  a  subsequent  paragraph. 

2.  It  may  seem  incredible  to  the  student  that  the  ponderous  globe  is  flying  through 
space  at  the  rate  of  68,000  miles  an  hour,  or  some  80  times  as  swift  as  a  bullet;  but^. 
like  many  other  astonishing  facts  in  Astronomy,  its  truth  can  easily  be  demonstrated. 
The  diameter  of  a  circle  is  to  its  circumference  as  7  is  to  22  nearly.    The  earth's  dis- 
tance from  the  sun  being  95,000,000  miles,  it  is  obvious  that  the  whole  diameter  of  her 
orbit   is  twice  that  distance,  or  190,000,000;   then,    as  7:  22::  190.000,000  :  597,142,857 
miles,  the  circumference  of  the  earth's  orbit.    Divide  this  sum  by  8,766,  the  number  of 
iiours  in  a  year,  and  we  have  68,108  miles  as  the  hourly  velocity  of  the  earth. 

3.  As  the  earth  is  not  propelled  by  machinery  like  a  steamboat,  or  borne  upon  wheels 
like  a  railroad  car,  it  is  not  strange  that  we  are  insensible  of  its  rapid  motion,  especially 
as  every  thing  upon  its  surface,  and  the  atmosphere  by  which  it  is  surrounded,  move 
9nward  with  ft  in  its  rapid  flight. 

CENTRIPETAL  AND  CENTRIFUGAL  FORCES. 

74.  The  mutual  attractive  force  of  the  sun  and  planets 
is  called  th#  centripetal  force ;  while  the  tendency  of  the 
planets  to  fly  off  from  the  sun,  as  they  revolve  around 
him,  is  called  the  centrifugal  force. 

1.  The  term  centripetal  is  from  centrum,  center,  and  petot  to  move  toward ;  and 
centrifugal,  from  centrum,  and  fugio,   to  fly  from  the  center. 

2.  The  centrifugal  force  is  generated  by  the  revolution  of  the  planet,  and  is  in  pro- 
portion to  its  velocity — the  more  rapid  the  revolution,  the  stronger  the  tendency  to  fly 
olf  from  the  sun. 

3.  If  the  centrifugal  force  were  suspended,  the  planets  would  at  once  fall  to  the  sun; 
and  if  the  centripetal  force  were  destroyed,  the  planets  would  fly  off  in  straight  lines, 
and  leave  the  solar  system  forever.     Then  might  be  realized  the  chaos  and  confusion  of 
the  poet : 

"  Let  Earth  unbalanced  from  her  orbit  fly, 
Planets  and  sunsrnn  lawless  through  the  sky; 
Let  ruling  angels  from  their  spheres  be  hurled, 
Being  OB  being  wreck'd,  and  world  on  world." 

75.  It  has  already  been  stated  (65),  that  the  force  of 
attraction  depends  somewhat  upon  the  distances  of  the 
attracting  bodies — those  nearest  together  being  mutually 

V3.  What  said  of  the  velocity  of  the  planets  ?  Of  the  earth  ?  (Table  ? 
Ktimarks  upon  it  ?  How  is  the  hourly  velocity  of  the  earth  ascertained  ? 
Why  not  sensible  of  this  rapid  motion?) 

74.  Centripetal  and  centrifugal  forces  ?  (Derivation  of  terms  ?  How  txm- 
force  generated  ?  Suppose  either  suspended  ?) 


46  ASTRONOMY. 


attracted  most.  It  follows,  therefore,  that  Mercury  ha* 
the  strongest  tendency  toward  the  sun,  Yenus  next,  this 
Earth  next,  &c.,  till  we  get  through  to  Neptune  ;  and  as 
the  centrifugal  force  which  is  to  balance  the  centripetal  is 
created  by  the  velocity  or  projectile  force  of  the  planets, 
that  velocity  must  needs  be  in  proportion  to  their  dis- 
tances, respectively,  from  the  sun  ;  the  nearest  revolving 
the  most  rapidly.  This  we  find  to  be  the  actual  state  of 
things  in  the  solar  system. 

The  mechanism  of  the  solar  system  strikingly  displays  the  wisdom  of  the  great 
Creator.  The  centrifugal  force  depends,  of  course,  upon  the  rapidity  of  the  revolution  ; 
and  in  order  that  these  forces  might  be  exactly  balanced,  God  has  imparted  to  each 
planet  a  velocity  just  sufficient  to  produce  a  centrifugal  force  equal  to  that  of  its  gravita- 
tion. Thus  they  neither  fall  to  the.  sun  on  the  one  hand,  nor  fly  off  beyond  the  reach  of 
his  beams  on  the  other,  but  remain  balanced  in  their  orbits  between  these  two  great 
forces,  and  steadily  revolving  from  age  to  age.  "  How  manifold  are  thy  works !  in  wis- 
dom hast  thou  made  ttieni  all." 

LAWS  OF  PLANETARY  MOTION. 

76.  Three  very  important  laws,  or  principles,  governing 
the  movements  of  the  planets,  were  discovered  by  Kep- 
ler, a  German  astronomer,  in  1609.     In  honor  of  their 
discoverer,  they  are  called  Kepler's  Laws. 

Kepler  was  a  disciple  of  Tycho  Brahe,  a  noted  astronomer  of  Denmark,  and  WHS 
equally  celebrated  with  his  renowned  tutor.  His  residence  and  observatory  were  in 
Wittenberg,  Germany. 

77.  Tiie  first  of  these  laws  is,  that  the 
orbits  of  all  the  planets  are  elliptical, 
having  the  sun  in  the  common  focus. 

The  point  in  a  planet's  orbit  nearest 
the  sun  is  called  the  perihelion  point, 
and  the  point  most  remote  the  aphelion 
point.  Perihelion  is  from  peri,  about  or 
near,  and  lielios,  the  sun ;  and  aphelion, 
from  apo,  from,  and  Tielios,  the  sun. 

From  this  first  law  of  Kepler,  it  results  that  the  plan- 
ets move  with  different  velocities,  in  different  parts  of 
their  orbits.  From  the  aphelion  to  the  perihelion  points, 
the  centripetal  force  combines  with  the  centrifugal  to 
accelerate  the  planet's  motion ;  while  from  perihelion  to 

75.  Why  the  planets  nearest  the  sun  revolve  most  rapidly  in  their  orbits  . 
(Remark  ?) 

76.  Laws  of  planetary  motion  ?    (Who  was  Kepler  ?} 

77.  State  the  first  of  Kepler's  laws.    Perihelion?    Aphelion* 


LAWS     OF     PLANETARY     MOTION. 


RADIUS  VICTOR. 

-e- 


aphelion  points,  the  centripetal 
acts  against  the  centrifugal  force, 
and  retards  it. 

1.  From  A  to  B  in  the  diagram,  the  centrifugal  force, 
represented  by  the  line  C,  acts  with  the  tendency  to 
revolve,  and  the  planet's  motion  is  accelerated  ;  but 
from  B  to  A    the  same  force,  shown  by  the  line  D, 
acts  against  the  tendency  to  advance,  and  the  planet 
is  retarded.    Hence  it  comes  to.  Aphelion  with  its 
least  velocity,  and  to  Perihelion  with  its  greatest. 

2.  In   the  statement  of  velocities  on  page  45,  the 
mean  or  average  velocity  is  given. 

78.  The  second  law  is,  that  the 
radius  vector  of  a  planet  describes 
equal  areas  in  equal  times.    The 

radius  is  an  imaginary  line  joining  the  center  of  the 

sun  and  the  center  of  the  planet,  in  any  part  of  its  orbit. 

Vector    is    from    veho,    to   carry ; 

hence  the  radius  vector  is  a  radius 

carried  round.     By  the  statement 

that  it  describes  equal  areas  in  equal 

times,  is  meant  that  it  sweeps  over 

the  same  surface  in  an  hour,  when 

a  planet  is  near  the  sun,  and  moves 

swiftly,  as,  when  furthest  from  the 

sun,  it  moves  most  slowly. 

The  nearer  a  planet  is  to  the  sun,  the  more  rapid 
its  motion.  It  follows,  therefore,  that  if  the  orbit  of  a 
planet  is  an  ellipse,  with  the  sun  in  one  of  the  foci,  its 
rate  of  motion  will  be  unequal  in  different  parts  of 
its  orbit — swiftest  at  perihelion,  and  slowest  at  aphe- 
lion. From  perihelion  to  aphelion  the  centripetal  more  directly  counteracts  the  cen- 
trifugal force,  and  the  planet  is  retarded.  On  the  other  hand,  from  the  aphelion  to  the 
perihelion  point,  the  centripetal  and  centrifugal  forces  are  united,  or  act  in  a  similar 
direction.  They  consequently  hasten  the  planet  onward,  and  its  rate  of  motion  is  con- 
stantly accelerated.  Now  suppose,  when  the  planet  is  at  a  certain  point  near  its  peri- 
helion, we  draw  a  line  from  its  center  to  the  center  of  the  sun.  This  line  is  the  radius 
vector.  At  the  end  of  one  day,  for  instance,  after  the  planet  has  advanced  considerably 
in  its  orbit,  we  draw  another  line  in  the  same  manner  to  the  sun's  center,  and  estimate 
the  area  between  the  two  lines.  At  another  time,  when  the  planet  is  near  its  aphelion, 
we  note  the  space  over  which  the  radius  vector  travels  in  one  day,  and  estimate  its  area. 
On  comparison,  it  will  be  found,  that  notwithstanding  the  unequal  velocity  of  the  planet, 
and  consequently  of  the  radius  vector,  at  the  two  ends  of  the  ellipse,  the  area  over 
which  the  radius  vector  has  traveled  is  the  same  in  both  cases.  The  same  principle  ob- 
tains in  every  part  of  the  planetary  orbits,  whatever  may  be  their  ellipticity  or  the  mean 
distance  of  the  planet  from  the  sun;  hence  the  rule,  that  the  radius  vector  describe* 
equal  areas  in  equal  times.  In  the  preceding  cut,  the  twelve  triangles,  numbered  1,  2, 
8,  «fcc.,  over  each  of  which  the  radius  vector  sweeps  in  equal  times,  are  equal. 

79.  The  third  law  of  Kepler  is,  that  the  squares  of  th& 


78.  State  the  second  law  of  planetary  motion.    Define  radius  vector, 
plain  this  serond  law.) 


ASTRONOMY. 


periodic  times  of  any  two  planets  are  proportioned  to  the 
cubes  of  their  mean  distances  from  the  sun. 

1.  Take,  for  example,  the  earth  and  Mars,  whose  periods  are  S35'25fi4  and  6S6-9796 
days,  and  whose  distances  from  t'.ie  sun  are  in  the  proportion  of  1  to  1-52369,  and  it  will 
be  found  that  (365-2564)* :  (6S6-9796)* ::(!)':  (1-52369)3. 

2.  According  to  these  laws,  which  are  known  to  prevail  throughout  the  solar  system, 
many  of  the  facts  of  astronomy  are  deduced  from  other  facts  "previously  ascertained. 
They  are,  therefore,  of  great  importance,  and  should  be  studied  till  they  are,  at  least, 
thoroughly  understood,  if  not  committed  to  memory. 


The 


MARS    IN    CONJUNCTION 

— e 

6 


ASPECTS   OF   THE   PLANETS. 

80.  By  the  aspects  of  the  planets  is  meant  their  ppsi 
tions  in  their  orbits  with  respect  to  each  other, 
principal  aspects 
are  conjunction, 
quadrature,  and  op- 
position. Two  bod- 
ies are  in  conjunc- 
tion when  in  the 
same  longitude ; 
that  is,  on  the  same 
north  and  south 
line  in  the  heavens. 
The  sign  for  con- 
junction is  6 .  When 
90°  apart,  bodies 
are  said  to  be  in 
quadrature,  with 
the  sign  n ;  and 
when  180°  apart,  or 
in  opposite  parts  of 
the  heavens,  they  are  in  opposition,  and  the  sign  is  §. 
Conjunctions  are  of  two  kinds.  An  inferior  conjunction 
is  when  the  planet  is  between  the  earth  and  the  sun ; 
and  a  superior  conjunction,  when  it  is  beyond  the  sun. 

1.  Let  the  student  imagine  himself  stationed  upon  the  earth  in  the  cut  Then  the 
eun  and  three  planets  above  are  in  conjunction.  The  inferior  and  superior  are  distin- 
guished :  while  at  A,  a  planet  is  shown  in  quadrature,  and  at  the  bottom  of  the  cut  the 
i»ianet  Mars  in  opposition  with  the  sun  and  interior  planet 

79.  State  the  third  law.     (Illustration  ?    What  said  of  the  importance  of  a 
knowledge  of  these  laws  ?) 

80.  What  meant  by  the  aspects  of  the  planets*    State  principal  aspects? 
"Define  each.  "Signs  ?     I  low  many  kinds  of  conjunctions  ?     Define  each. 
(•Explain  by  diagram.    When  is  Venus  nearest  ?    What  difference  at  superior 
taiid  inferior  conjunctions  ?) 


SIDEREAL   AND   SYNODIC   REVOLUTIONS.  49 


2.  "When  at  her  superior  conjunction,  Venus  is  154  millions  of  miles  from  the  earth  • 
but  when  at  her  inferior  conjunction,  she  is  only  26  millions  of  miles  distant,  or  the  whole 
diameter  of  her  orbit  nearer. 


SIDEKEAL   AND   SYNODIC   REVOLUTIONS. 

81.  The  sidereal  revolution  of  a  planet  is  a  complete 
revolution  from  any  given  point  in  its  orbit  around  to  the 
same  point  again. 

Sidereal,  from  sideralis — a  revolution  as  measured  by  the  stars.    See  page  28,  note  1. 
The  periodic  revolutions  of  the  planets,  given  at  Art.  71,  are  sidereal  revolutions. 

82.  A  synodic  revolution  is  from  one  conjunction  to 
the  same  conjunction  again. 

1.  The  term  synod  signifies  a  meeting  or  convention  ;  and  the  synodic  revolution  of 
a  planet  is  a  meeting  revolution :  that  is,  from  one  meeting  or  conjunction  to  another. 

2.  The  difference  between  a  sidereal  and  synodic  revolution  may  be  illustrated  by  the 
motion  of  the  hands  of  a  clock  or  watch.    At  twelve  o'clock,  the  hour  and  minute  hands 
are  together ;  but  at  one  o'clock,  when  the  minute-hand  has  made  a  complete  revolu- 
tion, and  points  to  XII.  again,  the  hour-hand  has  gone  forward  to  I.,  and  the  minute- 
hand  will  not  overtake  it  till  about  five  minutes  afterward.    The  revolution  of  the 
minute-hand  from  XII.  to  XII.  again,  represents  the  sidereal  revolution  of  a  planet; 
and  when  it  overtakes  tha  hour-hand,  it  becomes  a  synodic  revolution. 

3.  The  sidereal  and  synodic  periods  of  the  principal  planets  are  as  follows : 

Sidereal.  Synodic. 

Mercury —   88  days 115  days. 

Venus —      225    "  594 

Mars 1  year,  822    «  T80 

Jupiter 11    "      307    "  399 

Saturn 29    "      175    "  378 

Uranus 84    "        27—  367£ 

Neptune 164    "      226    "  867£ 

From  this  table  it  is  seen  that  the  synodic  periods  of  the  more  distant  planets  corre- 
spond very  nearly  with  the  periodic  time  of  the  earth.  Being  remote  from  the  sun,  they 
move  very  slowly,  and  the  earth  coon  overtakes  them,  after  performing  her  periodic 
revolution. 

SYNODIC  PERIODS  OF  THE  EXTEKIOE  PLANETS, 


Suppose  the  earth  and  Uranus  to  be  in  conjunction,  as  shown  at  A  B.  In  865J  days, 
the  earth  performs  her  sidereal  or  periodic  revolution,  and  return1}  to  the  point 
A  again.  La  the  mean  time  Uranus,  whose  periodic  time  is  84  years,  has  passed 

81.  What  meant  by  the  sidereal  revolution  of  a  planet  ?    (Derivation  of 
term?    Are  the  periods  of  the  planets  sidereal  revolutions  ?) 

82.  Synodic  revolution?     (Illustrate  difference  by  clock.     What  fact  re- 
specting synodic  periods  of  distant  planets  ?    How  explained  ?    Illustrate  by 
diagram.) 


50 


ASTRONOMY. 


through  only  ^  th  part  of  his  orbit,  or  about  4p  to  the  point  C ;  and  in  4^  days  ihe 
earth  overtakes  him  on  the  line  I).  It  is  on  this  account  that  the  synodic  period  o' 
Uranus  is  only  307^  days,  or  4^  days  longer  than  the  periodic  time  of  the  earth. 


THE   ECLIPTIC,    ZODIAC,    SIGNS,    ETC. 

83.  The  Ecliptic  is  the  plane  of  the  earttts  orbit,  or 
the  path  in  which  the  sun  appears  to  revolve  in  the 
heavens. 


1.  In  the  above  cut,  an  attempt  is  made  to  represent  the  ecliptic,  or  plane  of  th* 
earth's  orbit.     It  is  an  oblique  view,  which  makes  the  orbit  appear  elliptical.     It  shows 
one-half  of  the  snn  and  half  the  earth  on  one  side,  ami  half  on  the  other.     The  circle 
projecting  beyond  the  orbit  is  to  represent  the  plane  of  the  ecliptic,  indefinitely  ex- 
tended. 

2.  If  the  student  has  any  difficulty  in  getting  a  correct  idea  respecting  the  ecliptic, 
let  him  suppose  the  orbit  of  the  earth  to  be  a  hoop  of  small  wire  laid  upon  a  table:  the 
Burface  of  the  table,  both  within  and  without  the  hoop,  would  then  represent  the  plane 
of  the  ecliptic.     From  the  above  definition  and  description,  it  will  be  seen  that  the  eclip- 
tic passes  through  the  center  of  the  earth,  and  the  center  of  the  sun;  consequently  the 
ecliptic  and  the  apparent  path  of  the  sun  through  the  heavens  are  in  the  same  plane. 
It  will  be  easy,  therefore,  to  ascertain  the  true  position  of  the  ecliptic  in  the  heavens,  and 
to  imagine  its  course  among  the  stars. 

3.  The  plane  of  the  earth's  orbit  is  called  the  ecliptic,  because  eclipses  of  the  sun 
»nd  moon  never  take  place  except  when  the  imma  is  in  or  near  this  plane. 

84.  The  position  of  the  ecliptic  to  persons  north  of  the 
equator  is  south  of  us.  It  runs  east  and  west,  cutting 
the  centers  of  the  sun  and  earth.  North  of  the  ecliptic 
is  called  above  it;  and  south  of  it,  "below  it. 

The  student  should  again  be  reminded  that  there  is  no  absolute  up  nr  down  In  th* 
universe.  He  must  also  guard  against  the  idea  that  the  ecliptic  may  be  horizontal.  This 
term  has  reference  only  to  the  earth,  and  is  descriptive  of  a  plane  depending  altogether 
for  its  own  position  upon  that  of  the  observer,  as  shown  and  illustrated  at  20.  Though 
the  ecliptic  is  a  permanent  plane,  and  cuts  the  starry  heavens  around  us  at  the  same 
points  from  age  to  age,  it  has  no  absoJute  up  or  down,  unless  it  should  be  tlw  direction 
to  and  from  the  sun.  The  distinction  of  above  &T\<\  beltncis,  merely  arbitrary,  and  grows 
out  of  onr  position  north  of  the  equate  r,  which  aiakes  the  south  side  of  the-  ecliptic  ap- 
pear down  to  u& 


83.  What  the  ecliptic?    (How  cnt  the  earth  and  snn  ?    Point  out  its  course 
in  the  heavens.     Why  called  the  ecliptic?) 
ft4.  What  meant  by  abov*  and  below  the  ecliptic  ?    (Remarks  in  note.  > 


ECLIPTIC,  ZODIAC,  SIGNS,  ETC.  51 


85.  The  Poles  of  the  Ecliptic  are  the  extremities  of 
an  imaginary  axis   upon  which  the  ecliptic   seems  to 
revolve. 

As  the  ecliptic  and  equinoctial  are  not  in  the  same  plane,  their  poles  do  not  coincide, 
or  are  not  in  the  same  points  in  the  heavens.  The  cause  of  this  variation  will  be  ex- 
plained hereafter. 

86.  The  Zodiac  is  an  imaginary  belt  16°  wide,  viz., 
8°  on  each  side  of  the  ecliptic,  and  extending  from  west 
to  east  quite  around  the  heavens.     In  the  heavens,  it  in- 
cludes the  sun's  apparent  path,  and  a  space  of  eight  de- 
grees south,  and  eight  degrees  north  of  it. 


THE  KCLTPTIO  AND   ZODIACS. 


In  this  cut,  the  interior  dotted  circle  represents  the  earth's  orbit;  the  exterior  the 
pl<me  of  hor  orbit  extended  to'  the  starry  heavens.  The  dark  lines  each  side  cf  the 
ecliptic  are  the  limits  of  the  zodiac.  The  earth  is  shown  in  perspective,  largest  near  to 
us,  and  growing  smaller  as  her  distance  is  increased.  The  arrows  show  her  direction. 

87.  The  great  circle  of  the  zodiac  is  divided  into  twelve 
equal  parts,  called  signs.  (These  divisions  are  shown  in  the 
above  cut,  by  the  spaces  between  the  perpendicular  lines 
that  cross  the  zodiac.)  The  ancients  imagined  the  stars 
of  each  sign  to  represent  some  animal  or  object,  and  gave 
them  names  accordingly.  On  this  account,  they  gave 
the  name  zodiac  to  this  belt  around  the  heavens  ;  not,  as 
some  have  imagined,  because  it  was  a  zone,  but  from  the 
Greek  zoon,  an  animal,  because  so  many  animals  were 
represented  within  its  limits. 


85.  The  poles  of  the  ecliptic  ?    (Do  the  poles  of  the  eclipb'3  and  the  pole* 
of  heavens  coincide,?) 

86.  What  is  the  zodiac  f 

87.  How  is  the  zodiac  divided  ?    Idea  of  the  ancients  1    Origin  of  tha 
name  zodiac  ? 


52 


ASTRONOMY. 


88.  The  names,  order,  and  symbols  of  the  twelve  signs 
of  the  zodiac  are  as  follows : 


Aries  (or  the  Ram) 

Taurus  (the  Bull) 

Gemini  (the  Twins). . . . 

Cancer  (the  Crab) 

Leo  (the  Lion) 

Virgo  (the  Virgin) 


Libra  (the  Balance) 

Scorpio  (the  Scorpion)  .  f 
Sagittarius  (the  Archer) 
Capri cornus  (the  Goat) 
Aquarius  (theWaterman) 
Pisces  (the  Fishes)  . . . . 


ANCIENT  A8TEOLOGT. 


These  names  being  from  the  Latin,  their  signification  is  added  in  parentheses,  and  should 
>e  understood  by  the  pupil.  In  reciting,  however,  it  is  only  necessary  to  give  the  first 
names — as  Aries,  Taurus,  Gemini,  &c.  By  carefully  observing  these  symbols,  the  stu- 
dent will  detect  a  resemblance  between  several  of  them  and  the  objects  they  represent 
For  instance,  the  sign  for  Aries  represents  his  horns  ;  so  also  with  Taurus,  &c. 

89.  The  ancients  pretended  to  predict  future  events  by 
the  signs,  aspects,  &c.  This  art,  as  it  was  called,  was 
denominated  Astrology.  Astrology  was  either  natural 
or  judicial.  Natural ^  Astrology  aimed  at  predicting  re- 
markable occurrences  in  the  natural  world,  as  earthquakes, 
volcanoes,  tempests,  and  pestilential  diseases.  Judicial 
Astrology  aimed  at  foretelling 
the  fates  of  individuals  or  of  em- 
pires. 

"This  science,"  says  Webster,  "was  formerly  in 
great  request,  as  men  ignorantly  supposed  the  heaven- 
ly bodies  to  have  a  ruling  influence  over  the  physical 
and  moral  world  ;  but  it  is  now  universally  exploded 
by  true  science  and  philosophy."  A  fragment  of  this 
ancient  superstition,  like  the  adjoining  figure,  may 
still  be  met  with  occasionally  in  the  pages  of  an  al- 
manac ;  and  there  are  still  persons  to  be  found  in  al- 
most every  community  who  think  certain  "signs" 
govern  certain  portions  of  the  human  body,  and  that 
it  is  very  important  to  do  everything  "when  the 
sign  is  right"  Impostors,  also,  are  still  taking  advan- 
tage of  this  credulity;  and,  professing  to  "tell  for- 
tunes," as  they  call  it,  by  the  stars,  impose  upon  and 
defraud  the  ignorant  The  stars  have  no  more  to  do 
with  our  "  destiny"  than  we  have  with  theirs. 


90.  The  order  of  the  signs  is  from  west  to  east  around 
the  heavens.  Thus  Aries,  Taurus,  Gemini,  &c.,  around 
to  Pisces. 


88.  Names  of  the  signs  ?    Symbols  on  blackboard. 

&».  "What  is  astrology?    How  divided?    Define  each.     (Remark  of  Web- 
ster?    Of  the  author  ?) 
90:  The  order  of  the  signs  ?    (Describe  the  cut.    What  said  of  Taurus  'h 


CELESTIAL   LATITUDE   AND    LONGITUDE.  53 


PKllP£NIHCei.AK   VIEW   OF  THE   ECLIPTIC. 


OU      081 


On  pages  50  and  51.  we  presented  dbliqiie  views  of  the  ecliptic.  The  above  is  a  p» 
peniUcidar  view.  The  sun  is  seen  in  the  center,  and  the  earth  revolving  around  hin  ; 
and  in  the  distance  is  shown  the  circle  of  the  starry  heavens.  This  circle  is  divider! 
into  twelve  equal  parts,  representing  the  twelve  signs ;  while  the  object  which  the  stars 
in  each  sign  were  supposed  to  resemble  is  placed  in  that  sign,  and  the  m/mJ>ol  iinme* 
diately  opposite  and  within  the  sign.  But  the  head  of  Taurus  should  point  east  instead 
of  west. 

CELESTIAL   LATITUDE   AND   LONGITUDE. 

91.  Celestial  Longitude  is    distance  east  of  a  given 
point  in  the  heavens,  reckoned  on  the  ecliptic.     Begin- 
ning at  the  Vernal  .Equinox,  it  is  reckoned  eastward  to 
360°,  or  to  the  point  whence  we  staHed. 

The  pupil  will  consult  the  preceding  cut,  in  which  the  longitude  is  marked  for  every 
ten  degrees.  By  holding  the  book  up  to  the  south  of  him,  the  surface  of  the  page  will 
represent  the  plane  of  the  ecliptic ;  and  the  reckoning  of  10,  20,  30,  &c.,  from  the  top  of 
the  cut  eastward,  will  answer  to  the  manner  in  which  celestial  longitude  is  reckoned 
eastward  around  the  heavens. 

92.  Celestial  Longitude  is  either  Heliocentric  QY  Geo- 
centric.   The  heliocentric  longitude  of  a  planet  is  its 
longitude  as  viewed  from  the  sun  ;  and  the  geocentric,  its 
longitude  as  viewed  from  the  earth. 

Geocentric  is  from  ge,  the  earth,  and  kentron,  center;  and  fitMoesntric  from  heftos, 
the  sun,  and  kentron,  center. 

01.  Celestial  longitude  ?  Where  begin  to  reckon  ?  Ulustxt^  by  book. 
Point  out  order  of  reckoning  in  the  heavens. 

92.  What  is  heliocentric  longitude ?  Geocentric*  (l>onvation  if  terms? 
Illustrate  by  diagram.; 


ASTRONOISIY. 


GEOCENTRIC  AND  HELIOCENTRIC  LONGITtlUB. 


fn  this  cut,  the  planet  B,  when  viewed  from  the  earth  at  A,  seems  to  be  ;n  the  sign 
C2 ;  but  when  viewed  from  the  sun,  it  appears  to  be  in  n.  Again :  when  at  C,  her 
apparent  longitude  from  the  earth  is  in  il], ;  when  from  the  sun,  she  appears  to  be  in  t . 
The  learner  will  not  only  perceive  the  difference  between  geocentric,  and  7u>lioeent-ria 
longitude,  but  will  see  why  the  latter  more  than  the  former  indicates  the  true  position 
of  the  planet  It  is  an  easy  thing,  however,  if  one  is  known,  to  deduce  the  other  from  it. 


MEAN  AND   TRUE   PLACES 
OF   A   PLANET. 

93.  The  mean  place 
of  a  planet  is  the  place 
it  would  have  occupied 
had  it  revolved  in  a  cir- 
cular orbit,  and  with 
uniform  velocity. 

The  true  place  is  that 
which  it  really  occupies, 
revolving  as  it  does  in   M, 
an  elliptical  orbit,  and 
with  unequal  velocity. 

1.  In  the  cut,  the  dotted  ellipse 
represents  the  orbit  of  the  planet, 
and  the  points  T  T  T,  &c.,  its  true 
place.  In  the  circle  or  hypothetical 
orbit,  the  points  M  M,  Arc.,  indicate 
the  mean  place  of  the  planet. 


MEAN  AND  TRUE  PLACES  OF  A  PLANET 

T 


M 


What  is  meant  by  the  mean  place  of  a  planet  ?   The  true  place  f   (When 


DIRECT  AND   RETROGRADE   MOTIONS. 


55 


DIKECT  AND   RETROGRADE  MOTIONS. 


2.  From  the  perihrtion  to  the  aphelion,  it  will  be  seen  that  the  true  pbre  is  in  ad- 
vmice,  or  eattimrd,  of  the  mean  place ;  white  from  aphelion  to  perihelion  again,  th« 
mean  place  is  in  advance  of  the  true.    But  at  the  perihelion  and  aphelion  points,  the 
uiean  and  true  places  coincide. 

3.  In  one  respect,  the  cut  conveys  an  erroneous  impression,  as  it  represents  the  planet 
«s  passing  over  an  equal  distance  in  its  orbit  in  equal  times.    This  is  not  the  fact    The 
difference  in  its  velocity  in  different  parts  of  its  orbit  could  not  well  be  represented  here ; 
but  the  student  will  find  it  beautifully  illustrated  by  the  second  cut  on  page  47,  and  in 
fee«xpbuutorjr  note  accompanying  it 

94:.  Celestial  Latitude  is  distance  north  or  south  of 
the  ecliptic,  and  is  reckoned  to  the  pole  of  the  ecliptic, 
or  to  90°. 

DIRECT   AND   RETROGRADE   MOTIONS. 

95.  The  apparent  motion  of  a  planet  is  said  to  be 
direct  when  it  is  eastward  among  the  stars,  and  retrograde 
when  it  seems  to  go  back  or  westward  in  the  ecliptic. 
When  it  seems  to  move  neither  east  nor  west,  it  is  said  to 
be  stationary. 

96.  The  cause  of  the  appa- 
rent retrogression  of  the  in- 
terior   planets    is    the    fact 
that  they  revolve  much  more 
rapidly  than  the  earth,  from 
which  we  view  them ;  causing 
their  direct  motion  to  appear 
to  be  retrograde. 

Suppose  the  earth  to  be  at  A,  and  Venus  at 
B,  she  would  appear  to  be  at  0,  among  the 
stars.  If  the  earth  remained  at  A  while  Ve- 
nus was  passing  from  B  to  D,  she  would 
seem  to  retrograde  from  C  to  E ;  but  as  the 
earth  passes  from  A  to  F  while  V«nus  goes 
from  B  to  D,  Venus  will  appear  to  be  at  (i ; 
arid  the  amount  of  her  apparent  westward 
motion  will  only  be  from  0  to  Gr. 

97.  The     apparent     retro- 
grade motions  of  the  exterior 
planets  is  due  to  the  rapidity 

with  which  the  position  from  which  we  view  them  is 

is  the  true  in  advance  of  the  mean?    When  the  reverse?    When  do  they 
coincide  ?     Wherein  is  the  cut  defective  ?     Where  have  we  a  true  rcpreseu 
tiition  ?) 

94.  Cetestisl'latifadt  f    How  reckoned? 

9;"..  When  is  a  planet's  apparent  motion  direct?  Retrograde?  When  is 
a  planet  said  to  be  stationary? 

%.  State  the  cause  of  the  apparent  retrogression  of  an  interior  planet, 
(illustrate  by  diagram,) 

y7.  The  cause  of  retrogression  of  exterior  planets.  (Illustrate  by  diagram- 
What  feet,  show u  0 


56 


ASTRONOMY. 


changed,  as  we  are  carried  rapidly  through  space  with  the 
earth,  in  her  annual  journey  around  the  sun. 

98.  The  portion  of  the  ecliptic  through  which  a  planet 
seems  to  retrograde  is  called  the  Arc  of  Retrogradation. 
The  more  remote  the  planet  the  less  the  arc,  and  the 
longer  the  time  of  its  retrogression. 

RETROGRADE  MOTION   OF  THB  BXTKEIOB  PLATTETS. 


:B 

i: 


1.  Suppose  the  earth  at  A,  and  the  planet  Neptune  at  B,  he  would  then  appear  to 
be  at  C,  among  the  stars ;  but  as  Neptune  moves  but  a  little  from  B  toward  F,  while  the 
earth  is  passing  from  A  to  D,  Neptune  will  appear  to  retrograde  from  C  to  E.     What- 
ever Neptune  may  have  moved,  however,  from  B  toward  F,  will  go  to  reduce  the 
amount  of  apparent  retrogression. 

2.  It  is  obvious  from  this  figure,  that  the  more  distant  an  exterior  planet  is,  and  the 
•lower  it  moves,  the  less  will  be  its  arc  of  retrogradation,  and  the  longer  will  it  be  retro- 
grading.   Neptune  appears  to  retrograde  180  days,  or  nearly  half  the  year. 

The  following  table  exhibits  the  amount  of  arc  and  the  time  of  the  retrogradation  of 
the  principal  planets: 

Arc.  Day» 

Mercury 13^° 23 

Venus *. 16  42 

Mars  16  73 

Jupiter 10  121 

Saturn 6  139 

Uranus 4  151 

Neptune 1  180 

99.  The  greatest  elongation  of  an  interior  planet  is  the 
greatest  apparent  distance  east  or  west  of  the  sun  at 
which  it  is  ever  found. 

In  the  second  cut  back,  the  point  B  would  represent  the  greatest  eastern,  and  D  the 
greatest  western,  elongation  of  the  planet  At  these  two  points  she  would  appear  to  be 
stationary. 

100.  The  greatest  elongations  of  Yenus  vary  from  45° 
to  480.     The  fact  that  she  never  departs  more  than  48° 
from  the  sun  proves  that  her  orbit  is  within  that  of  the 
earth ;  and  the  variation  in  her  elongations  shows  that 
her  orbit  is  not  an  exact  circle. 

98.  What  meant  by  the  Arc  of  Retrogradation? 

99.  Greatest  elongation  ? 

100.  Greatest  elongation  of  Venus !    What  does  it  prove  ? 


VENUS   AS   MORNING  AND  EVENING  STAE.  57 

101.  When  Venus  is  west  of  the  sun,  and  risp.s  before 
him,  she  is  morning  xtar  •  and  when  east  of  the  s\u\  »!  e 
is  evening  star. 


VENUS    AS    HORNING    AND    EVENING    STAR. 
M 


r\\/ 

\    '  '  />X 


J! 

C 


-    • 


1.  Let  the  student  hold  the  book  up  south  of  him,  and  he  will  at  once  see  why  Venus 
is  alternately  morning  and  evening  star.    Let  the  plane  A  B  represent  the  sensible  or 


visible  horizon,  C  D  the  apparent  daily  path  of  the  sun  through  the  heavens,  and  E 

?ition.      The  sun  is  shown   at  three  different  po 
namely,  rising  in  the  east,  o'n  the  meridian,  and  setting  in  the  west;  while  Venus  is 


seen  revolving  around  him  from  west  to  east,  or  in  the  direction  of  the  arrows.  Now 
it  is  obvious  that  when  Venus  is  at  F,  or  went  of  the  sun,  she  sets  before  him  as  at  G, 
and  rises  before  him  as  at  H.  She  must,  therefore,  be  morning  star.  On  the  other 
hand,  when  she  is  east  of  the  sun,  as  at  J,  she  lingers  iu  the  west  after  the  sun  has  gone 
down,  as  at  K,  and  is  consequently  evening  star.  ' 

2.  In  this  cut,  Yenus  would  be  at  her  greatest  elongation  eastward  at  J,  and  ^cest~ 
ward  at  F,  and  in  both  cases  would  be  "  stationary."    At  L  and  M  she  would  be  in 
conjunction  with  the  sun. 

3.  Were  the  earth  to  suspend  her  daily  rotation,  with  the  sun  on  the  meridian  of  the 
observer,  as  represented  at  L,  we  might  readily  watch  Venus  through  her  whole  circuit 
around  the  sun. 

4.  Venus  may  sometimes  be  seen  at  mid-day,  either  east  or  west  of  the  sun,  and  Dr. 
Dick  considers  the  day-time  most  favorable  for  observing  her  with  a  telescope. 

102.  Venus  is  morning  and  evening  star,  alternately, 
for  about  292  days,  or  from  one  conjunction  to  another. 
Appearing  first  east  and  then  west  of  the  sun,  she  was 
regarded  by  the  ancients  as  two  different  stars,  which  they 
called  Phosphor  and  Hesperus. 

When  Venus  is  near  her  greatest  elongation  from  the  sun,  she  is  one  of  the  most  beau- 
tiful stars  in  the  heavens.  She  is  very  easily  found,  either  just  before  sunrise,  or  just 
after  sundown;  and  we  earnestly  recommend  the  class  to  ascertain  where  she  is,  at  the 
time  of  learning  this  lesson,  and  to  watch  her  movements  for  a  few  months,  and  see  if 
tliey  do  not  correspond  with  the  description  here  given.  The  knowledge  acquired  will 
thus  be  located  in  the  ueavens. 

101.  When  morning  and  when  evening  star  1 

102.  How  long  is  Venus  alternately  morning  and  evening  star  1    How  re 
?arded  by  the  ancients  \    (Remark  in  note  0 


58  ASTRONOMY. 


103.  The  greatest  elongations  of  Mercury  vary  from  1G 
to  29  degrees  from  the  sun,  which  proves  his  orbit  to  be 
elliptical,  and  to  be  within  that  of  Venus. 

1.  As  Mercury  never  departs  more  than  29°  from  the  sun.  when  at  his  greatest  elonga- 
lion,  and  Venus  is  never  nearer  than  about  45°,  when  at  her  greatest  elongation,  it  is 
evident  that  his  orbit  is  inside  that  of  Venus. 

2.  When  at  perihelion,  Mercury  is  only  29,305,000  miles  from  the  sun's  center ;  while 
in  the  opposite  part  of  his  orbit,  or  in  aphelion,  he  reaches  to  44,474,000 — making  a  vari- 
ation of  distance,  arising  from  the  ellipticity  of  his  orbit,  of  more  than  15,169,000  miles, 
which  is  nearly  five  times  as  great  as  in  the  case  of  the  earth. 

104.  In  consequence  of  the  nearness  of  Mercury  to  the 
sun,  he  is  very  rarely  seen ;  and  if  seen  at  all,  it  must  be 
in  strong  twilight,  either  morning  or  evening.     He  never 
appears  conspicuous,  even  under  the  most  favorable  cir- 
cumstances, but  twinkles  like  a  star  of  the  third  magni- 
tude, with  a  pale  rosy  light. 

By  consulting  an  almanac,  the  student  can  ascertain  -when  Mercury  is  at  his  greatest 
elongation,  and  if  it  is  eastward,  look  out  for  him  low  down  in  the  west,  just  after  sun- 
set If  his  elongation  is  westward,  he  must  be  looked  for  in  t/te  east,  before  sunrise. 
It  will  be  worth  rising  early  to  see  him. 

DEVIATION  OF  THE  ORBITS  OF  THE  PLANETS  FROM  THE  PLANE 
OF  THE  ECLIPTIC. 

105.  Although  the  sun  is  the  great  center   around 
which  all  the  planets  revolve,  it  should  be  borne  in  mind 
that  no  two  <,f  them  revolve  in  the  same  plane.     Taking 
the  plane  of  the  earth's  orbit  or  ecliptic  as  the  standard, 
the  orbits  of  the  other  planets  all  depart  from  that  plane, 
some  more  and  some  less.     As  a  consequence,  they  all 
pass  through  or  cut  the  plane  of  the  ecliptic  twice  at 
every  revolution. 

VENUS  PASSING  AND  BEPASStNG  THE  PLANK  OF  THB  EARTIl'S  DEBIT. 


In  this  cut,  the  space  included  within  the  orbit  of  the  earth  is  tinted  to  represent  » 
plane.    Within  her  orbit,  and  part  above,  and  part  below  it,  may  be  seen  the  orbit  of 

103.  Greatest  elongation  of  Mercury  ?    Proves  what  ?    (Show  how  demon- 
strated.    What  said  of  the  eccentricity  of  the  orbit  of  Mercury?) 

104.  Is   Mercury  often  seen?     When,  if  at  all?     Appearance?     (How 
find  ?) 

105.  Are  all  the  planetary  orbits  in  the  same  plane  ?    "What  consequence 
follows  ? 


DEVIATION   OF   THE   ORBITS   OF   THE   PLANETS. 


7("»'is.  the  arrows  showing  her  direction.    Her  orbit  goes  out  of  sight  when  it  oassea 
the  plane  of  the  ecliptic. 

106.  The  points  where  a  planet  passes  the  plane  of  the 
ecliptic  are  called  the  Nodes  of  its  orbit.  They  are  in 
opposite  sides"of  the  ecliptic,  and  of  course  180°  apart. 
The  point  where  they  pass  south  of  the  ecliptic  is  called 
the  descending  node,  and  marked  £3 ;  and  that  through 
which  they  pass  north  of  the  ecliptic  is  called  the  ascend- 
ing node,  and  marked  &.  The  Line  of  the  Nodes  is  a 
]ine  drawn  from  one  node  to  the  other  across  the  ecliptic. 

The  nodes,  ascending  and  descending,  and  their  symbols,  acdalso  the  line  of  the  nodes, 
marked  L  N,  are  all  well  represented  in  the  preceding  cut. 

INCLINATION   OF  THE  ORBITS   OF  THK  PLANETS  TO  THE  PLANE  OF  THE  ECLIPTIC. 


107.  The  nodes  of  the  planetary  orbits  are  not  all  in 
the  same  longitude,  but  are  distributed  all  around  the 
ecliptic.  In  astronomical  works  and  calculations,  the 
longitude  of  the  ascending  node  only  is  noted,  as  the 
opposite  node  is  always  just  180°  from  it. 

The  longitude  of  the  ascending  nodes  of  the  planets,  respectively,  is  as  follows : 


75 

Hebe  . 

188 

Pallas 

173 

Earth  

Parthenope 

Hygeia  .  . 

Mars 

.   .        48 

Jupiter 

93 

Flora 

110 

141 

112 

Clio  

Irene  

Uranus  .  . 

72 

Testa 

103 

130 

Iris 

2GO 

171 

106.  What  arc  the  Nodes  of  a  planet's  orbit  ?     How  situated  with  respect  to 
each  other  ?     What  called  respectively,  and  why  ?    What  meant  by  the  lint 
if  the  nodes  ? 

107.  Are  the  nodes  of  all  the  planetary  orbits  in  the  same  longitude  ?    How 
distributed  ?    Which  node  usually  mentioned  and  located  ?    Why  not  both  ? 


60  ASTRONOMY. 


108.  The  deviation  of  the  planets,  respectively,  from 
the  ecliptic  varies  from  1°  18"  to  34£°.  The  orbits  of  the 
larger  planets  are  all  near  the  ecliptic,  while  some  of  the 
asteroids  depart  widely  from  it.  On  this  account  they  are 
sometimes  called  ultra-zodiacal  planets. 

The  preceding  cut  may  help  the  student  to  fonn  an  idea  of  the  inclination  of  the 
planetary  orbits;  but  we  must  guard  against  the  impression  it  may  make  that  all  the 
planetary  nodes  are  in  the  same  part  oftfie  ecliptic,  as  we  were  obliged  to  represent  in 
the  cut.  Instead  of  tliis,  they  are  distributed  all  about  the  ecliptic.  Again :  the  cut 
shows  the  several  planets  at  about  the  same  distance  from  the  sun,  contrary  to  the  fact, 
as  stated  and  illustrated  on  page  30.  The  dotted  line  represents  the  earth's  orbit,  or 
plane  of  the  ecliptic,  and  the  other  lines  the  planes  of  the  orbits  of  several  of  the  plan- 
ets, and  their  departure  from  the  ecliptic.  The  inclination  of  the  several  orbits  's,  ia 
round  numbers,  as  follows: 


Mercury 7° 

Venus 3023' 

Earth — 

Mars lost' 

Flora 5053' 

Clio 7°  08' 

Vesta 7008' 

Iris 5023' 


Metis 5034' 

Hebe 14047' 

Parthenope . . . — 

Egcria — 

Astraea 5»  19' 

Irene — 

Eunomia — 

Juiio 130    3' 


Ceres 10°  871 

Pallas 34037' 

Hygeia 


Jupiter 1°  18" 

Saturn SP  W 

Uranus 1°  46' 

Neptune 1°  46' 


OF   TRANSITS. 

109.  The  passage  of  a  heavenly  body  across  the  me- 
ridian of  any  place,  or  across  the  disk  of  the  sun,  is 
called  a  transit.     A  planet  will  seem  to  pass  over  the 
disk  of  the  sun  when  it  passes  directly  between  us  and 
him  ;  and  as  none  but  the  interior  planets  can  ever  get 
between  us  and  the  sun,  it  is  obvious  that  no  others  can 
ever  make  a  transit  over  his  disk. 

The  te"/s  'transit  is  sometimes  used  with  reference  to  terrestrial  objects,  as  when  we 
speak  of  the  transit  or  passage  of  goods  through  a  country.  The  words  transition, 
transitive,  transitory,  &c.,  are  derived  from  the  primitive  word  transit. 

110.  Mercury  and  Venus  are  the  only  planets  that  can 
appear  to  cross  over  the  sun's  disk,  as  viewed  from  oui 
globe. 

"Were  we  stationed  upon  one  of  the  remote  exterior  planets,  we  might  see  the  earth, 
and  Mars,  and  Jupiter  transit  the  sun  ;  but  as  it  is,  we  shall  never  witness  such  phe- 
nomena, or,  at  least,  till  we  leave  the  present  world. 

111.  "Were  the  orbits  of  Mercury  and  Yenus  in  the 
same  plane  with  that  of  the  earth,  they  would  transit  the 

108.  To  what  extent  do  the  planetary  orbits  depart  from  the  ecliptic? 
What  said  of  the  larger  planets  ?    Of  the  smaller  ?    (Remarks  upon  the  cut. 
State  the  inclination  of  Mercury,  Venus,  Mars,  &c.) 

109.  What  is  a  transit  ?    When  do  planets  transit  the  sun  ?    What  planets 
do  this  ?    Why  not  the  exterior  ?    (Remarks  upon  term  transit.) 

110.  What  planets  make  transits  across  the  sun's  disk?     (Remarky  \K 
aote. ) 


TRANSITS. 


61 


sun  at  every  synodic  revolution  ;  but  as  one-half  of  eac"b 
of  their  orbits  is  above,  and  the  other  half  below  the 
ecliptic,  they  generally  appear  to  pass  either  above  01 
below  the  sun. 


B 
..-•-> 

-A*- 
...... 


Let  the  right  line  A,  joining  the  earth  and  the  sun  in  the  above  diagram,  represent 
the  plane  of  the  ecliptic.  Now  when  an  interior  planet  is  in  this  plane,  as  shown  at  A, 
it  may  appear  to  be  upon  the  sun's  disk;  but  if  it  is  either  above  or  below  the  ecliptic, 
as  shown  at  B  and  C,  it  will  appear  to  pass  either  above  or  below  the  sun,  as  shown  at 
D  and  E. 

112.  A  transit  can  never  occur  except  when  the  inte- 
rior planet  is  in  or  very  near  the  ecliptic.  The  earth  and 
the  planet  must  be  on  the  same  side  of  the  ecliptic  ;  the 
planet  being  at  one  of  its  nodes,  and  the  earth  on  the 
line  of  its  nodes. 

PHILOSOPHY  OP  TBAXSrrS. 


lliis  cut  represents  the  ecliptic  and  zodiac,  with  the  orbit  of  an  interior  planet,  h* 
nodes,  &c.  The  line  of  his  nodes  is,  as  shown,  in  the  16°  of  b  and  the  16°  of  fll 
Now  if  the  earth  is  in  » ,  on  the  line  L  N,  as  shown  in  the  cut,  when  Mercury  is  at 
his  ascending  node  (Q),  he  will  seem  to  pass  upward  over  the  sun's  face,  like  a  dark 
spot,  as  represented  in  the  figure.  On  the  other  hand,  if  Mercury  is  at  his  y  when 
the  earth  is  in  the  16°  of  m,,  the  former  will  seem  to  pass  downward  across  the  disk  ol 
the  sun. 

113.  As  the  nodes  of  the  planetary  orbits  are  in  oppo- 

111.  Why  not  transits  every  revolution  of  Mercury  and  Venus?    (Illus- 
trate by  diagram.) 

112.  When  must  transits  occur,  if  at  all  ?     Where  must  the  earth  und 
planet  be  ?     (Illustrate  by  diagram.) 


ASTRONOMY. 


site  sides  of  the  ecliptic,  it  follows  that  the  earth  must 
pass  the  line  of  the  nodes  of  the  interior  planets,  re- 
spectively, in  opposite  months-  of  the  year.  These 
months  are  called  the  node  months  of  the  planet,  and  are 
the  months  in  which  all  its  transits  must  occur. 

114.  In   making   transits   across  the  sun's   disk,   the 
planets  seem  to  pass  from  east  to  west,  and  to  ascend  or 
descend,  as  respects  the  ecliptic,  according  as  the  planet 
is  at  the  ascending  or  descending  node. 

This  variation  in  the  direction  of  the  planets,  during  different  transits,  is  well  repre- 
sented in  the  next  cut. 

115.  The  node  months  of  Mercury  are  May  and  No- 
vember. 

All  the  transits  of  Mercury  ever  noticed  have  occurred  in  one  or  the  other  of  thes* 
months,  and  for  the  reason  already  assigned.  The  first  ever  observed  took  place 
November  6,  1631;  since  which  time  there  have  been  29  others  by  the  same  plaiiet — 
iu  all  30—8  In  May,  and  22  in  November. 

116.  The  last  tran- 
sit  of  Mercury   oc- 
curred     November 
11,   1861;   and  the 
next  will  take  placo 
November  4,  1868. 
Besides    this,   there 
will  be  four  more  dur- 
ing the  present  cen- 
tury— two  in   May, 
and  two  in  Nov'r. 

The  accompanying  cut  is  a  de- 
lineation of  all  the  transits  of 
Mercury  from  1802  to  the  close 
of  the  present  century.  The 
dark  line  running  east  and  west 

across  the  sun's  center  represents  SOUTH 

the  plane  of  the  ecliptic,  and  tho 
dotted  lines  the  apparent  paths 

of  Mercury  in  the  several  transits.  The  planet  is  shown  at  its  nearest  point  to  the  sun's 
center.  His  path  in  the  last  transit  and  in  the  next  will  easily  be  found. 

2.  The  last  transit  of  Mercury  was  observed  in  this  country  by  Professor  Mitchel.  at 
the  Cincinnati  Observatory,  and  by  many  others  both  in  America  and  in  Europe. 


TRANSITS   OF  MERCURY. 

&ORTH 


113.  What  are  the  node  months  f    (Explain  by  diagram.) 

114.  In  what  direction  do  planets  cross  the  sun  in  transits,  and  why? 

115.  Which  are  the  node  months  of  Mercury  ? 

116.  When  did  the  last  transit  of  Mercury  occur  ?    When  will  the  next 
tftke  place  ?    (What  represented  in  the  cut.  ?    "Describe.     Where  is  the  planet 
•Itown  ?    What  said  of  last  transit  of  Mercury  ?) 


TRANSITS.  63 


Che  writer  had  made  all  necessary  f  reparation  for  observing  the  phenomenon  at  his 
residence,  near  Oswego,  New  York ;  but,  unfortunately,  his  sky  was  overhung  with 
clouds,  which  hid  the  sun  from  his  view,  and  disappointed  all  his  hopes. 

117.  The  node  months  of  Venus  are  December  and 
June.     The  line  of  her  nodes  lies  in  Gemini  (IE)  and 
Sagittarius  (^);  and  as  the  earth  always  passes  those 
points  in  the  months  named,  it  follows  that  all  transits  of 
V  enus  must  occur  in  those  months  for  ages  to  come. 

This  proposition  will  be  well  understood  by  consulting  the  cut  on  page  61  ;  for  as  the 
ineof  Venus's  nodes  is  only  one  sign  ahead  of  that  of  Mercury,  the  earth  will  reach 
.hat  point  in  the  ecliptic  in  one  month  after  she  pasy.es  the  line  of  Mercury's  nodes;  so 
.hat  if  his  transits  occur  in  May  and  November,  hers  should  occur  in  June  and  Decem- 
uer,  as  is  always  the  case. 

118.  The  last  transit  of  Venus  occurred  June  3,  1769  ; 
and  the  next  will  take  place  December  8,  1874. 

1.  Only  three  transits  of  Venus  have  as  yet  been  observed — namely,  December  4,  1639 ; 
June  5,  l?bl ;  and  June  3,  17G9.     it  is  said  that  Itittenhouse  was  so  interested  in  view- 
ing that  of  1769.  that  he   actually  fainted.     In  defining  the  term  transit,  Dr.  Webster 
says:  "I  witnessed  the  transit  i>f  Venus  over  the  sun's  di^k,  June  3,  ITb'O."    (See  '•  Una- 
bridged" Dictionary.)    The  next  four  will  occur  December  8,  1»74;  December  5,  l&jii; 
June  7,  2004  ;  and  June  5,  2012. 

2.  The  first  transit  ever  witnessed  was  that  of  December  4,  1639.    The  observer  was 
a  young  man  named  Horrox.  living  in  an  obscure  village  near  Liverpool,  England.    The 
table  of  Kepler,  constructed  upon  the  observations  of  Tycho  Brahe,  indicated  a  transit  of 
Venus  in  1631,  but  none  was  observed.     Horrox,  without  much  assistance  from  books 
and  instruments,  set  himself  to  inquire  into  the  error  of  the  tables,  and  found  that  such 
a  phenomenon  might  be  expected  to  happen  in  1639.     lie  repeated  his  calculations 
during  this  interval  with  all  the  carefulness  and  enthusiasm  of  a  scholar  ambitious  of 
being  the  first  to  predict  and  observe  a  celestial  phenomenon  which,  from  the  creation  of 
the  world,  had  never  been  witnessed.     Confident  of  the  result,  he  communicated  his  ex- 
pected triumph  to  a  confidential  friend  residing  in  Manchester,  and  desired  him  to  watch 
for  the  event,  and  to  take  observations.    So  anxious  was  Horrox  not  to  fail  of  witnessing 
it  himself,  that  he  commenced  his  observations  the  day  before  it  was  expected,  arid  re- 
sumed them  at  the  rising  of  the  sun  on  the  morrow.     But  the  very  hour  when  his  cal- 
culations led  him  to  expect  the  visible  appearance  of  Venus  on  the  sun's  disk,  was  also 
t/'te  appointed  hour  for  the  public  worship  of  God,  ontlie  Sabbath,  The  delay  of  a  few 
minutes  might  deprive  him  forever  of  an  opportunity  of  observing  the  transit.     If  its 
very  commencement  were  not  noticed,  clouds  might  intervene,  and  conceal  it  until  the 
sun  should  set ;  and  nearly  a  century  and  a  half  would  elapse  before  another  opportunity 
would  occur.    He  had  been  waiting  for  the  event  with  the  most  ardent  anticipation  for 
eight  years,  and  the  result  promised  much  benefit  to  the  science.    Notwithstanding  all 
(Att,  Horrox  twice  suspended  his  observation*,  and  twice  repaired  to  the  house  of  God, 
the  great  Author  of  the  bright  works  he  delighted  to  contemplate.    When  his  duty  wad 
chus  performed,  and  he  had  returned  to  his  chamber  the  second  time,  his  love  of  scienco 
was  gratified  with  full  success,  and  he  saw  what  no  mortal  eye  had  observed  before.     If 
any  thing  can  add  interest  tp  this  incident,  it  is  the  modesty  with  which  the  young 
astronomer  apologizes  to  the  world  for  suspending  his  observations  at  all.     "  I  observed 
it,"  says  lie,  ''from  sunrise  till  nine  o'clock,  again  a  little  before  ten.  and  lastly  at  noon, 
and  from  on*  to  two  o'clock ;  the  rest  of  the  day  being  devoted  to  higher  duties,  which 
might  not  be  neglected  for  these  pastimes." 

3.  The  transit  of  1769  was  observed  with  intense  interest  by  astronomers  in  both  hemi- 
spheres.    To  secure  the  advantages  of  observations  at  different  points,  Capt.  Cook  was 


117.  Node  months  of  Venus  ?    Where  line  of  nodes  ?    Why  June  and 
December  her  node  months  ?     ( Why  only  one  month  after  those  of  Mercury  ?) 

118.  When  last  transit  of  Venus  ?    Next  ?    (How  many  have  been  ob- 
served ?    What  said  of  Kittenhousr- '    Webster?    When  next  four  transits 
of  Venus  ?    When  first  transit  notivx ,«  .    What  said  of  it?    That  of  1769 

— use  of  observations  ?) 


64: 


ASTRONOMY 


?ent  to  the  Pacific  in  the  bark  "Endeavor,"  where  he  perished  subsequently  by  the 
hands  of  savages  at  one  of  the  Sandwich  islands.  Observations  upon  these  transits  fur- 
nish data  for  important  astronomical  calculations. 

119.  In  consequence  of  the  earth's  annual  revolution 
around  the  sun,  he  appears  to  travel  eastward,  through 
all  the  signs  of  the  zodiac,  every  365J  days.  It  is  this 
eastward  motion  of  the  sun  that  causes  the  stars  to  rise 
and  set  earlier  and  earlier  every  night. 

BUN'S  APPAKENT  MOTION   AROUND  THE   ECLIPTIC. 


i^et  a  person  walk  around  a  tre«,  for  instance,  at  a  short  distance  from  it,  and  it  will 
appear  to  sweep  around  the  horizon  in  an  opposite  direction.  So  as  the  earth  revolves 
annually  about  the  sun,  the  sun  appears  to  traverse  the  circle  of  the  heavens  in  the  oppo- 
site direction.  Suppose  the  earth  is  at  A  on  the  20th  of  March"*,  the  sun  will  appear  to 
be  at  B  in  the  opposite  side  of  the  ecliptic.  As  the  earth  moves  on  in  her  orbit  from  A 
to  C,  the  sun  will  appear  to  move  from  B  to  D ;  and  will  seem  thus  to  traverse  the 
whole  circle,  of  the  heavens  every  365.5:  days,  or  as  often  as  the  earth  revokes  around 
him.  The  time  of  the  sun's  apparent  entrance  into  the  different  constellations  as  he  jour- 
neys eastward,  is  usually  laid  down  in  almanacs.  Thus:  "Sun  enters  T  (Anes)  20th  of 
March,  &c.;"  at  which  time  the  earth  would  enter  the  sign  ^  (Libra),  and  the^ura 
would  seem  to  enter  the  opposite  sign  Aries. 


119.  What  said  of  sun's  apparent  motion  ?  Cause?  Time  of  revolution  ? 
Effect  upon  the  stars?  (Illustration  from  tree?  By  diagram.)  What  is 
n:  eaut  by  the  sun's  entering  Aries  ?  When  2  Where  earth  theiV  / 


PRIMARY    PLANETS.  65 


CHAPTER    II. 


PRIMARY     PLANETS     CONTINUED. 

120.  BESIDES  the  revolution  around  the  sun,  the  planets 
all  revolve  rapidly  about  their  respective  axes,  as  they 
perform  their  celestial  journeys.      This  is  called  their 
diurnal  revolution. 

The  evidences  of  the  earth's  revolution  have  already  been  considered  on  pages  13  and 
14.  That  most  of  the  other  planets  revolve  has  been  ascertained  by  carefully  observing 
the  motions  of  spots,  as  they  seemed  to  pass  periodically  over  their  disks. 

121.  The  axis  of  the  earth  is  inclined  to  the  plane  o^ 
the  ecliptic  23°  28'.     It  is  always  parallel  to  itself— that 
is,  it  always  inclines  the  same  way,  and  to  the  same 
amount. 

INCLINATION  OF  THE  EABTH'B  AXIS  TO  THE  PLANE  OF  THE  ECLIPTIC. 

PLANE  OP        ^£5!>k         THE  ECLIPTIC.  /<5fi£ 


1.  The  inclination  of  the  earth's  axis,  and  its  parallelism  to  itself;  are  exhibited  in  the 
above  cut,  as  also  in  the  cuts,  pages  50,  51,  and  &4,  to  which  the  student  will  do  well  to 
turn. 

2.  The  author  is  aware  that  the  poles  of  the  earth  have  a  slow  motion  around  the  pole 
of  the  ecliptic,  requiring  25,000  years  for  a  single  revolution,  but  prefers  to  consider  this 
point  hereafter,  in  connection  with  the  precession  of  the  equinoxes. 

122.  The  axes  of  all  the  planets  are  inclined  more  or 
less  to  the  planes  of  their  respective  orbits.  This  incli- 
nation, so  far  as  known,  is  as  follows  : 


Venus     ...     75° 
Mars  .  28°  42 


Jupiter    ...       3°  05' 
Saturn  26°  50' 


120.  What  revolution  have  the  planets  besides  around  the  sun  ?    What 
called  ?    (What  proof  of  the  earth's  revolution  ?    Of  the  other  planets  ?) 

121.  What  said  of  the  axis  of  the  earth  ?    Of  the  stability  of  its  inclina- 
tion ?    (Is  there  no  variation  ?) 

122.  Are  the  axes  of  the  other  planets  inclined  ?    To  what  extent,  respect- 
ively?   (Substance  of  note  1  ?    Illustrate  by  diagram.    Note  2  ?) 


ASTRONOMY. 


1.  The  student  will  bear  in  mind  that  the  above  inclination  is  not  to  the  eclipti<\  or 
plane  of  the  earth's  orbit,  but  to  the  plane  of  the  orliits  of  the  several  planets  respect- 
ively. Take  the  case  of  Venus,  for  instance : 


The  orbit  of  Venus  departs  from  the  ecliptic  8£°,  as  stated  at  108,  while  her  axis  is  in- 
clined to  the  plane  of  her  orbit  75°,  as  shown  in  the  above  figures.  This  distinction 
should  be  kept  definitely  in  view  by  the  student. 

2.  The  inclination  of  the  axes  of  the  several  planets,  each  to  the  plane  of  its  own  or- 
bit, is  represented  in  the  following  cut : 

INCLINATION  OF  THE  AXES  OF  THE  SEVERAL  PLANETS  TO  THB  PLANES  OF  THEIE  CEBITS. 


,  123.  The  inclination  of  the  earth's  axis  to  the  plane  of 
the  ecliptic  causes  the  equinoctial  to  depart  23°  28'  from 
the  ecliptic.  This  angle  made  by  the  equinoctial  and  the 
elliptic  is  called  the  Obliquity  of  the  Ecliptic. 


OBLIQUITY  OF  THE  ECLIPTIC. 

A 


jLe<  the  line  A  A  represent  the  axis  of  the  earth,  and  B  B  the  poles  or  axis  of  the  ecTip- 
Mc..  Now  if  the  line  A  A  inclines  toward  the  plane  of  the  ecliptic,  or.  in  other  words, 
departs  from  the  line  B  B  to  the  anvtm.i  of  23°  28',  it  is  obvious  that  the  plane  of  the 

123.  What  effect  has  the  inclination  of  the  earth's  axis  upon  the  equinoc- 
tial ?  What  is  the  obliquity  of  the  ecliptic  ?  (Illustrate  by  diagram.) 


EQUINOCTIAL   AND   SOLSTITIAL   POINTS.  67 


equator,  or  equinoctial,  will  depart  from  the  ecliptic  to  the  same  amount   This  depart- 
ure, shown  by  the  angles  OC,  constitute  the  obliquity  of  the  ecliptic. 

'  124.  The  permanent  inclination  of  the  earth's  axis,  and 
her  revolution  around  the  sun,  cause  first  one  pole  to  be 
enlightened  and  then  the  other,  thus  producing  the  sea- 
sons. The  same  inclination  and  revolution  cause  the  sun 
to  appear  to  oscillate  from  north  to  south,  crossing  the 
equator  twice  every  year.  This  is  called  the  surfs  decli- 
nation. (See  page  26.) 

This  subject  of  the  seasons  will  be  sufficiently  understood  by  examining  the  cuts  on 
pages  64  and  65. 

125.  The  equinoctial  points  in  the  earth's  orbit  are  two 
points  in  opposite  sides  of  the  ecliptic,  at  which  the  sun 
is  exactly  in  the  equinoctial ;   or,  in  other  words,  the 
plane  of  the  equinoctial  exactly  cuts  the  sun's  center. 
The  first  of  these  is  passed  on  the  20th  of  March  (the 
sun  beginning  then  to  decline  northward),  on  account  of 
which  it  is  called  the  vernal  equinox  /  and  the  other  on 
the  23d  of  September,  on  account  of  which  it  is  called 
the  autumnal  equinox.     (See  the  earth  at  A  and  B,  in 
the  cut,  page  64.) 

If  the  sun  is  vertical  at  the  equator,  be  will,  of  course,  shine  to  both  poles,  as  repre- 
sented in  the  cut,  and  the  days  and  nights  will  be  equal  all  over  the  world.  Hence  the 
name  equinoctial,  from  the  Latin  cequus,  equal,  and  nvx,  night. 

126.  The  solstitial  points  are  those  points  in  the  earth's 
orbit  where  the  sun  ceases  to  decline  from  the  equinoc- 
tial, and  begins  again  to  return  toward  it.     They  are 
respectively  90°  from  the  equinoctial  points. 

The  Summer  Solstice  is  reached  on  the  21st  of  June, 
when  the  sun  has  the  greatest  northern  declination,  and 
it  is  summer  in  the  northern  hemisphere. 

The  Winter  Solstice  is  reached  on  the  23d  of  Decem- 
ber, when  the  sun  has  the  greatest  southern  declination, 
and  it  is  summer  in  the  southern  hemisphere,  and  winter 
m  the  northern.  (See  the  earth  at  E  F,  cut,  page  64.) 

124.  "What  other  effects  from  the  inclination  of  the  earth's  axis  ?    Sun's 
declination  ? 

125.  What  are  the  equinoctial  points  f    How  distinguished,   and  why ! 
When  passed  ?    (Substance  of  note  ?) 

126.  The  solstitial  points  ?    How  far  frcm  the  equinoctial  points  ?    How 
distinguished  ?     When  passed  ? 


68 


ASTRONOMY. 


127.  The  amount  of  the  sun's  declination  north  and 
south  of  the  equinoctial  is  23°  28' ;  answering  to  the  in- 
clination of  the  earth's  axis,  by  which  it  is  caused,  and 
marking  the  limits  of  the  tropics  upon  the  earth's  surface. 

1.  On  the  21st  of  June  the  sun  reaches  his  greatest  northern  declination,  or  Summer 
Solstice,  and  is  vertical  on  the  Tropic  of  Cancer.    From  this  tim^  he  approaches  the 
equator  of  the  heavens  till  the  20th  of  September,  when  he  crosses  it,  and  begins  to  de- 
cline southward.    On  the  23d  of  December  he  has 

reached  his  greatest  southern  declination,  or  Winter      SHADOWS  AT  THE  EQUATOR. 

Solstice,  and  begins  to  return  toward  the,  equinoctial, 

which  he  passes  on  the  20th  of  March,  and  reaches  his 

Summer  Solstice  again  on  the.  21st  of  June.    In  this 

manner  he  continues  to  decline,  first  north  and  then 

south  of  the  equator,  from  year  to  year.     But  it  should 

not  be  forgotten  that  the  sun  does  not  really  move,  ^ 

first  north  and  then  south,  but  that  the  apparent  mo-  g 

tion  is  caused  simply  by  the  inclination  of  the  earth's  ^/' 

axis  and  her  revolution  around  the  sun.  s  / 

2.  The  sun's  declination  may  be  easily  measured 
by  the  shadow  of  a  suitable  object  upon  the  earth's 
surface.     Suppose  the  flag-staff  in  the  cut  to  stand  / 
perpendicularly,   and  exactly  on   the  equator.      On      ^— 

the  23d  of  December  the  shadow  would  be  thrown  C  B  A 

•northward  to  A,  or  23°  28'— just  as  far  as  the  sun  has 

declined  south.  At  12  o'clock,  on  the  20th  of  March,  and  the  23d  of  September,  there 
would  be  no  shadow;  and  on  the  21st  of  June,  it  would  extend  southward  23°  28'  to 
C.  Thus,  at  the  equator,  the  shadow  falls  first  north  and  then  south  of  all  perpendicular 
objects,  for  six  mouths  alternately. 


23-28' 


MEASURING  THE  SUN  8  DECLINATION  IN  NORTHERN  LATITUDE. 


3.  This  cut  shows  how  the  student  may  measure  the  sun's  declination  wherever  lie 
ttiaybs  located  north  of  the  equator.  The  shadows  are  such  as  are  cast  by  objects 
during  the  year,  about  45°  north  of  the  equator.  On  the  23d  of  December,  when  the 
sun  has  his  greatest  declination,  the  shadow  of  the  flag-staff  extends  north  at  12 
o'clock  to  the  point  C,  where  two  boys  are  seen,  having  just  driven  down  a  stake. 
From  this  time  to  June  21st  the  shadow  gradually  shorten*,  till  on  that  day  it  reaches 
the  point  B,  where  another  stake  is  driven.  It  then  begins  to  elongate,  and  in  six 
months  is  extended  to  C  again.  The  point  A  is  just  half-way  from  B  to  C  in  angular 
measurement,  though  the  distances  on  the  plain  in  the  picture  are  very  different 
When  the  sun  is  on  the  equator,  March  21st  and  September  23d,  the  shadow  will  reach 
only  to  A ;  and  the  angle  A  B  and  the  top  of  the  statf  shows  the  northern,  and  A  C  and 
the  top  of  the  staff  the  southern  declination.  It  will  be  found  to  be  23°  2S'  each  way, 
as  marked  in  the  figure. 


127.  To  what  extent  does  the  sun  decline  from  the  equinoctial  north  and 
south?  Why  not  more?  (Substance  of  note  1  ?  Note  2,  and  explain  by 
•liagiam.  Note  3,  and  diagram.  What  is  a  gnomon  f) 


ROTATION  OF  THE  PLANETS  UPON  THEIR  AXES.  69 


4.  The  angle  formed  by  the  top  and  bottom  of  the  pole  and  the  point  A  will  exactly 
correspond  with  the  latitude  of  the  place  where  the  experiment  is  made. 

5.  Let  the  students  try  this  matter  for  themselves.     Select  a  level  spot,  and  put  up  a 
stake,  say  ten  feet  high.     Get  an  exact  "noon  mark,"  or  north  and  south  line,  whore 
the  stake  is  driven,  and  at  12  o'clock,  every  fair  day,  put  down  a  small  stake  at  the  end 
of  the  shadow.     In  this  manner  you  will  soon  be  able  to  measure  the  sun's  declination 
for  yourselves,  to  determine  the  latitude  of  the  place  where  >ou  live,  and  to  understand 
how  mariners  at  sea  ascertain  their  latitude  by  the  declination  of  the  sun. 

6.  The  ancients  had  pillars  erected  for  the  purpose  of  making  observations  upon  their 
shadows.    Such  a  pillar  is  called  a  gnomon. 

ROTATION  OF  THE  PLANETS  UPON  THEIR  AXES. 

128.  The  time,  so  far  as  known,  of  the  revolution  of 
the  planets  upon  their  respective  axes,  or,  in  other  words, 
the  length  of  their  natural  days,  is  as  follows  : 


h.      m. 


Mercury  ...  24     5 

Yenus      ...  23  21 

Earth  ....  24  00 

Mars    ,  24  37 


h.      m. 


Jupiter  .  .  .  9  56 
Saturn  .  .  .  10  29 
Uranus  ...  9  30 
Neptune  .  .  Unknown 


These  statistics  are  given  upon  the  authority  of  Sir  John  P.  W.  Herschcl,  though  he 
marks  Juno  and  Uranus  as  doubtful. 

129.  The  revolution  of  the  earth  upon  its  axis  is  the 
cause  of  the  agreeable  vicissitudes  of  day  and  night. 


PHILOSOPHY  OF  DAT  AND  NIGHT. 


How  wisely  adapted  to  the  happiness  of  His  creatures  are  all  the  works  of  God !  The 
night  prepares  us  for  the  day,  and  the  day  in  turn  prepares  us  to  welcome  the  night; 
and  in  both  instances  the  change  ministers  to  the  happiness  of  man  and  beast.  Andbut 
for  being  carried  around  into  the  darkness  of  the  earth's  shadow,  we  should  never  have 
admired  the  dazzling  firmament,  as  it  declared  the  glory  of  God,  and  showed  forth  his 
handiwork.  How  beautiful  the  poetic  allusion  to  the  revealing  power  of  night  I 

Mysterious  Night !  when  our  first  parent  knew 

Thee,  from  report  divine,  and  heard  thy  name, 

Bid  he  not  tremble  for  this  lovely  frame, 
This  glorious  canopy  of  light  and  blue  ? 
Yet,  'neath  a  curtain  of  translucent  dew, 

Bathed  in  the  rays  of  the  great  setting  flame, 

Hesperus  with  the  host  of  heaven  came ; 
And  lo !  creation  widened  in  men's  view. 

128.  In  what  time  do  the  other  planets  rotate  on  their  respective  axes ! 
f^ote?)  ' 

129.  Cause  of  day  and  night  ?    (Substance  of  note  ?    Poetic  quotation  ?) 


70  ASTRONOMY. 


Who  could  have  thought  such  darkness  lay  conceal'd 

Within  thy  beams,  O  Sun  !  or  who  could  find, 
Whilst  fly,  and  leaf,  and  insect  stood  revealed, 

That  to  such  countless  orbs  tliou  uiad'st  us  blind  ? 
Why  do  we,  then,  shun  death  with  anxious  strife? 
If  light  cau  thus  deceive  us,  may  not  life  ? 

130.  The  earth   and   all   the   other 
planets  revolve  eastward  upon   their 
axes,  or  in  the  same  direction  in  which 
they  revolve  in  their  orbits.      This 
also  is  determined  (with  the  exception 
of  the  earth)  "by  observing  the  motion 
of  spots  upon  their  surfaces,  by  the 
aid  of  telescopes. 

1.  In  the  cut  we  have  an  arc  of  the  earth's  orbit,  and  the 
earth  revolving  on  her  axis  as  she  revolves  around  the  sun. 
The  arrows  show  the  direction  in  both  cases. 

2.  By  holding  the  book  up  south  of  him,  and  looking  at- 
tentively at  the  cut,  the  student  will  understand  why  the 
sun  "rises'1  or  first  appears  in  the  east.     It  is  because  the 

earth  revolves  eastward.    Thus  the  observer  at  A  is  carried  round  into  the  light,  ami 
Bees  the  sun  rise  when  he  reaches  B. 

TIME. 

131.  Time  is  duration  measured  either  by  natural  or 
artificial  means.*   The  principal  natural  indicators  of  the 
lapse  of  duration  are  the  revolution  of  the  earth  upon  its 
axis,  marking  a  natural  day  ;  the  change  of  the  moon, 
denoting  a  lunar  month  ;  and  the  cycle  of  the  seasons, 
denoting  a  year.     Time  is  measured  artificially  by  clocks, 
watches,  chronometers,  dials,  &c. ;  the  standard  being 
the  solar  day  still,  which  is  divided  artificially  into  24 
parts,  called  hours,  and  these  again  into  minutes  and 
seconds. 

The  aboriginal  tribes  of  this  country  all  reckoned  time  by  "moons,"  or  months,  r" 
denoted  by  the  moon's  changes. 

132.  The  motion  of  the  earth  upon  its  axis  is  the  most 
regular  of  which  we  have  any  knowledge.     It  does  not 
vary  one  second  in  a  thousand  years. 

To  this  stability  of  the  earth's  motion  upon  her  axis  the  prophet  refers  when  he  says: 
'  Thus  tiaitii  the  Lord,  If  ye  can  break  my  covenant  of  the  day,  and  my  covenant  of  the 

130.  In  what  direction  do  the  planets  rotate  on  their  axes  ?    How  ascer- 
tained ?    (Explain  why  the  sun  appears  to.rise  in  the  east.) 

131.  What  is  timer    What  natural  standards  ?    Artificial?    (How  meas- 
ured by  aborigines  ?) 

132.  What  said  of  earth's  motior  on  axis?    (What  reference  to  in  Scriu- 


TIME.  71 


ar.dthat  there  should  not  be  day  and  nieht  in  their  seasons,  then  may  also  my 
cc  reliant  be  broken  with  David,"  &c.—Jeremiah  xxxiii.  "20. 

133.  Time  is  of  two  kinds — Solar  and  Sidereal.     A 
solar  day  is  the  time  elapsing  from  the  sun's  crossing  the 
meridian  of  any  place,  to  his  coming  to  the  same  me- 
ridian again.     A  sidereal  day  is  the  time  intervening 
between  the  transit  of  a  star  across  the  meridian,  to  its 
coming  to  the  same  meridian  again. 

134.  A  solar  day  consists  of  24  hours,  at  a  mean  rate, 
but  a  sidereal  day  is  accomplished  in  23  hours,  56  min- 
utes, and  4  seconds ;  the  solar  day  being  nearly  4  minutes 
longer.     This  slight  difference  of  about  4  minutes  daily, 
between  solar  and  sidereal  time,  amounts  to  one  whole 
day  in  every  365J  days.     Owing  to  the  revolution  of 
the  earth  around  the  sun,  and  his  apparent  annual  revo- 
lution eastward  among  the  stars,  it  requires  366  revolu- 
tions of  the  earth,  as  measured  by  the  fixed  stars,  to 
make  365-J-  days,  as  measured  by  the  sun. 

135.  The  cause  of  this  difference  in  the  apparent  revo- 
lutions of  the  sun  and  stars,  and  consequent  difference  in 
the  length  of  a  natural  day,  as  measured  by  the  passage 
of  a  star  or  of  the  sun  across  the  meridian,  is  this :  The 
earth  is  constantly  advancing  in  her  orbit  while  she  re- 
volves on  her  axis,  causing  the  suii  to  appear  to  move 
slowly  eastward  among  the  stars ;  or,  what  is  the  same 
thing,  the  stars  to  appear  to  rise  earlier  and  earlier  every 
night,  and  one  after  another  to  overtake  and  pass  by  the 
sun.     (See  Article  119.)     When,  therefore,  the  meridian 
is  brought  around  to  that  point  in  the  heavens  where 
the  sun  was  24  hours  before,  he  is  not  there,  but  has 
moved  a  little  eastward.     But  a  star  that,  24  hours  before, 
was  exactly  behind  the  center  of  the  sun  in  the  distant 
heavens,  will  be  found  west  of  the  sun,  and  will  conse- 
quently cross  the  meridian  before  the  sun  does.     The 
time  required  for  the  meridian  to  revolve  from  the  star 
to  the  sun  constitutes  the  3  minutes  56  seconds  difference 
between  solar  and  sidereal  time. 

133.  Kinds  of  time  ?    Define  oncli. 

1«4.  Length  of  solar  day  ?    Sidereal?    Difference?    Amount  in  year  ? 
185.  State  the  cause  of  the  difference  in  the  time  of  the  apparent  revolutioo 
of  the  sun  uud  stars.     Illustrate  hy  diagram. 


72  ASTKONOMY. 


BOLAE  AND   SIDEREAL  7IMK. 
v         Q  _______  ___  _____  SIDEREAL  DAY 


SUN  ON  THE   MERIDIAN 


1.  To  the  man  at  A  the  sun  (S)  is  exactly  on  the  meridian,  or  it  is  twelve  o'clock, 
aoon.  The  earth  passes  on  from  B  to  D,  and  at  the  same  time  revolves  on  her  axis. 
When  she  reaches  D,  the  man  who  has  stood  on  the  same  meridian  has  made  a  complete 
revolution,  as  determined  by  the  star  G  (which  was  also  on  his  meridian  at  twelve  o'clock 
the  day  before) ;  but  the  sun  is  now  east  of  the  meridian,  and  he  must  wa.it/our  minute* 
for  the  earth  to  roll  a  little  further  eastward,  and  bring  the  sun  again  over  his  north  and 
south  line.  If  the  earth  were  not  revolving  around  the  sun,  her  solar  and  sidereal  days 
would  be  the  same ;  but  as  it  is,  she  has  to  perform  a  little  more  than  one  complete  revo 
lution  each  solar  day,  to  bring  the  sun  on  the  meridian. 

EQUATION  OF  TIME. 

136.  As  the  distant  stars  have  no  motion,  real  or  ap- 
parent, around  the  ecliptic,  and  the  earth's  motion  upon 
it  is  uniform,  it  results  that  sidereal  time  is  always  exactly 
the  same. 

A  clock  that  keeps  sidereal  time  is  called  a  sidereal  do^Jc.  One  of  these  instruments 
is  almost  indispensable  in  the  observatory  of  the  astronomer. 

137.  Solar  time  is  constantly  varying.     No  two  suc- 
cessive solar  days  are  exactly  of  a  length.     The  24  hours 
given  as  the  length  of  a  solar  day  (134)  is  the  average 
of  all  the  solar  days  throughout  the  year.     Hence  it  is 
called  mean  solar  time.     The  time,  as  indicated  by  the 
transit  of  the  sun  across  the  meridian,  from  day  to  day, 
is  called  apparent  time. 

138.  A  well-regulated  clock  will  keep  mean  solar  time, 
and  will  vary  from  the  apparent  time  (as  indicated  by  a 
noon  mark,  or  dial)  to  the  amount  of  16J-  minutes  one 
way,  and  14J  the  other.     The  sun  will  at  one  time  cross 
the  meridian  16J  minutes  before  it  is  noon  by  the  clock — 
the  apparent  time  being  16  J  minutes  faster  than  mean  or 
clock  time ;  while  at  another  time  it  will  be  noon  by  the 
clock  14  J  minutes  before  it  is  noon  by  the  sun. 

136.  Is  sidereal  time  always  the  same ?    Why  must  it  be?    (What  is  a 
nidereal  clock  ?) 

137.  What  said  of  the  variations  of  solar  time  ?    What  is  mean  solar  time  ? 
Apparent  ? 

138.  What  time  do  ooumion  clocks  keep  ?    How  much  variation  from  sun  ? 
Cow? 


EQUATION   OF   TIME.  73 


139.  The  difference  bet  ween 'apparent  and  mean  solar 
time  is  called  the  Equation  of  Time.     It  is  greatest  about 
the  3d  of  November,  when  the  clock  is  16  minutes  and 
17  seconds  behind  the  sun.     Four  times  a  year — viz., 
April  15th,  June  15th,  September  1st,  and  December  23d 
— the  clock  and  sun  will  agree;  or,  in  other  words,  mean 
and  apparent  time  will  be  alike. 

140.  The  inequality  of  the  solar  days  depends  upon 
two  causes — the  unequal  velocity  of  the  earth  in  her 
orbit  (77, 78),  and  the  inclination  of  her  axis  to  the  plane 
of  her  orbit  (123). 

141.  If  the   earth's   crbit  were  an  exact   circle,  she 
would  move  with  the  same  EQirAL  80LAK  DATS. 
velocity  in  all  parts  of  it; 

and  if  she  revolved  with 
regularity  upon  her  axis,  * 

her  solar  days  would  be        ®- 
exactly  of  a  length.  ^ 

Let  the  circle  in  the  adjoining  cut  rep-        / 
resent  the  earth's  orbit,  and  the  projec-      «>^—    • 
tions  from  the  earth  toward  the  sun  a        ; 
pillar  or  gnomon  standing  upon  a  given      £)it~- 
meridian.    The  cut  will  then  show  that       ^^ 
with  a  circular  orbit,  and  uniform  motion          isy 
in  it,  and   a  regular  rotation  upon  her         1£p 
axis,  the  earth  would  bring  the  gnomon  V 

around  toward  the  sun  at  regular  inter-  ^ 

viils.  both  of  distance  in  her  orbit,  and 
of  time.  In  that  case,  all  apparent  solar 
days  would  be  equal. 

142.  As  the  orbit  of  the  earth  is  elliptical,  it  requires 
more  time  for  the  earth  to  pass  from  the  vernal  equinox, 
through  the  aphelion,  to  the  autumnal  equinox,  than  it 
does  from  the  autumnal  equinox,  through  the  perihelion, 
to  the  vernal  equinox.     The  difference  is  about  eight  days 
— the  sun  being  north  of  the  equinoctial  about  eight  days 
longer  than  he  is  south  of  it.     Hence  the  summers  of  the 
northern  hemisphere  are  longer  than  the  winters. 

143.  As  the  earth's  orbit  is  an  ellipse,  and  the  earth 

139.  What  this  difference  called  ?    When  greatest?    When  no  difference  J 

140.  What  causes  the  inequality  in  the  length  of  the  solar  days  ? 

141.  What  necessary  in  order  that  they  may  be  equal"?    (Illustrate  by  dia- 
gr;nn  nn<l  explanations.) 

143.  What  etf-pt  has  the  ellipticity  of  the  earth's  orbit  upon  the  length  of 
t  ae  seasons,  north  and  south  ot'  the' equator? 

4 


74  ASTRONOMY. 


moves  faster  in  some  parts  of  it  than  in  others,  while  ito 
rotary  motion  is  uniform,  it  follows  that  its  orbitual  ve- 
locity in  longitude  must 

V. .  ,     °    /.        ,  -,  ITNEQTTAL  SOLAR  DAYS. 

sometimes  be  faster,  and 
at  others  slower  than  its 
orbitual  motion,  thus  caus- 
ing an  inequality  in  the 
length  of  the  solar  days.  ^ 

From  A  to  B  in  the  adjoining  ent, 
the  orbitual  motion  is  slower  than  its  %j&- 

mean  rate,  and  the  rotary  motion  gains 
upon  it.     Hence  the  gnomon  is  shown        ;R(fe- 
revolving  too  fast,  and  appointing  east  V; 

of  the  sun,  when   the   earth  has  per-  V 

formed   her  journey  for  a  mean  solar  €p 

day.    From  B  to  A,  the  earth's  motion  \ 

in  her  orbit  gains  upon  her  rotary  mo- 
tion, and  the  gnomon  is  behind,  or  west 
of  the  sun.  At  A  and  B  the  clock 
and  sun  would  agree.  From  A  to  D 
the  sun  gains  on  the  clock,  till  it  gets  ^ 

1-U  minutes  ahead.     From  D  to  B  this  C 

difference  is  diminished,  till  at  B  the 

sun  and  clock  agree.  From  B  to  C  the  clock  gains  on  the  sun,  till  the  difference  is  Hfcj 
minutes;  and  from  C  to  A  this  difference  diminishes,  till  at  A  mean  and  apparent  time 
agree  again. 

144.  The  earth's  perihelion  is  in  n,  and  her  aphelion 
in  /  ;  the  first  of  which  she  passes  on  the  first  of  Janu- 
ary, and  the  latter  on  the  3d  of  July.     We  are  conse- 
quently about  three  millions  of  miles  nearer  the   sun 
Jan.  1,  than  July  3d. 

The  natural  effect  of  this  variation  would  be,  so  far  as  it  had  any  influence,  to  modify 
the  cold  and  heat  in  the  Northern  Hemisphere,  and  to  augment  both  in  the  Southern. 
For  instance,  our  nearness  to  the  sun  in  January  would  slightly  soften  our  winter,  while, 
»t  the  same  time,  it  slightly  increased  the  heat  of  the  snmmer  south  of  the  equator. 
So,  also,  our  increased  distance  in  July  would  diminish  the  heat  of  our  summer,  and  at 
the  same  time  enhance  the  cold  of  the  corresponding  winter  in  the  Southern  Hemf- 
Pphere.  But  the  variation  of  3TOOOrOOO  miles  is  so-  slight,  when  compared  with  the  whol« 
distance  of  the  sun,  that  the  change  of  temperature  produced  thereby  is  imperceptible. 

THE  CALENDAR,  LEAP  TEAE,  OLD  AND  NEW  STYLE,  ETC. 

145.  The  Julian  calendar  divided  the  year  into  12 
months,  containing  in  all  365  days.     But  a  full  astro- 
nomical year,  or  the  time  requisite  for  the  earth  to  re- 
volve from  one   equinox  around  to  the  same  equinox 
again,  consists  of  365d.  5h.  48rn.  51s.     Hence  the  Julian 

143.  Explain  the  cause  of  this  inequality  ?    'Illustrate  by  diagram.) 

144.  Where  are  the  perihelior  ir-  '    npuelion  points,  and  whan  passed? 
When  nearest,  and  how  inucu  f    (What  effect  £) 

145.  Describe  the  Julian  calendar?  "An  astronomical  year?    What  dJJ- 
f*)i  ence  ?    What  effect  ?    How  corrected  ? 


THE    CALENDAK,   LEAP   TEAK,    ETC.  75 


year  was  nearly  6  hours,  or  one  day  in  every  four  years, 
too  short;  which,  if  left  unconnected,  would  in  time  com- 
pletely reverse  the  seasons,  giving  harvests  in  January, 
and  snow  in  July.  To  prevent  this  constant  falling  be- 
hind, a  correction  was  applied,  by  adding  one  day  to 
February  every  fourth  year.  Hence  it  is  called  Bissex- 
tile or  Leap  Year. 

146.  But  one  whole  day  added  for  every  four  years 
was  44m.  36s.  too  much.     From  A.  D.  325  to  1582  this 
excess  amounted  to  about  10  days ;  so  that  the  civil  year 
was  thus  much  ahead  of  the  astronomical.     In  1582, 
Pope  Gregory  XIII.  applied  a  further  correction,  or  re- 
formed the  Julian  calendar.     To  make  the  civil  and  as- 
tronomical years  agree,  so  that  the  vernal  equinox  would 
happen  on  the  21st  of  March,  as  it  did  1257  years  before. 
Gregory  resolved  to  strike  out  of  the  civil  year  the  10 
days  it  had  gained,  and  ordered  that  the  5th  of  October 
should  be  called  the  15th.     This  reformed  or  corrected 
calendar  is  called  the  Gregorian  calendar. 

147.  To  prevent  the  civil  year  from  running  ahead  of 
the  astronomical  again,  in  the  lapse  of  centuries,  by  the 
llm.  12s.  which  it  exceeded  the  astronomical,  it  was  pre- 
scribed that  at  certain  convenient  periods  the  intercalary 
day  of  the  Julian  period  should  be  omitted.     Thus  the 
centennial  years  1700,  1800,  1900,  are,  according  to  the 
Julian  calendar,  bissextiles  ;  but  on  these  it  was  ordered 
that  the  intercalary  day  should  not  be  inserted;  inserted 
again  in  2000,  but  not  inserted  in  2100,  2200,  2300 ;  and 
so  on  for  succeeding  centuries. 

148.  The  Gregorian  or  reformed  calendar  was  adopted 
as  soon  as  promulgated,  in  all  Catholic  countries  ;  but  in 
England,  the  "change  of  style,"  as  it  was  called,  did  not 
take  place  till  September,  1752.     Eleven  nominal  days 
were  then  struck  out,  and  the  3d  of  September  was  called 
the  14th.     At  the  same  time,  the  time  of  the  beginning 

146.  Was  the  calendar  then  correct?    Why  not?    What  result?    Who 
corrected?     When?     How?     What  this  reformed  calendar  called  ? 

147.  What  further  correction  necessary  ?     How  effected  ? 

118.  Was  the  Gregorian  calendar  at  once  adopted  ?     When  in  England ! 
How  then  adopted  ?    What  other  change  at  the  same  time  ?    What  effect  in 


ASTRONOMY. 


NOON 


76 

of  tlie  civil  year  was  changed  from  the  25th  of  March  to 
January  1st,  as  it  now  stands.  The  year  1752,  which 
was  to  have  begun  on  the  25th  of  March,  was  made  to 
begin  on  the  1st  of  January  preceding  ;  so  that  for  dates 
falling  between  the  1st  of  January  and  the  25th  of  March, 
the  number  of  "the  year  is  one  greater  by  the  New  than 
by  the  Old  Style.  And  as  the  intercalary  day  was  omit- 
ted in  1800,  there  is  now,  for  all  dates,  12  days  difference 
between  the  old  and  new  styles.  Russia  is  now  the  only 
Christian  country  in  which  the  Gregorian  calendar  is  not 
used. 

TIME,  AS  AFFECTED  BY  LONGITUDE. 

149.  As  the  sun's  crossing  the  meridian  of  any  place 
determines  it  to  be  12  o'clock,  apparent  solar  time,  at 
that  place,  it  is  evident 

that  12  o'clock  comes 
sooner  to  places  east  on 
the  earth's  surface,  and 
later  to  places  west. 

1.  Let  the  adjoining  cnt  represent  the 
earth,  the  arrows  indicating  the  direc- 
tion of  her  revolution,  and  the  sun  being 
on  the  meridian  at  XII.  at  the  top.     It 
will  then  be  day  over  all  the  light  por- 
tion of  the  globe,  and  night  over  all  the 
shaded    portion.      On     the     meridian 
exactly  under  the  sun  it  is  jnst  XII. 
o'clock  noon  ;  while  at  the  meridian  on 
the  opposite  side  of  the  earth  it  is  just 
12  o'clock  at  night,  or  midnight.     When 
the  light  and  shade  meet  on  the  right,  it 
is  VI.  o'clock  morning;   and  directly 
opposite  on  the  left,  is  VI.  o'clock  even- 
ing. 

2.  Observe  that  when  it  is  XII.  at  A, 
it  is  I.  o'clock  at  B,  II.  o'clock  at  0,  &c., 

•while  it  is  only  XI.  o'clock  at  D,  X.  o'clock  at  E,  IX.  o'clock  at  F,  &c. ;  thus  showing 
Low  it  is  that  time  is  earlier  east,  and  later  west  of  any  given  meridian. 

150.  Every  15°  of  longitude  upon  the  earth's  surface 
makes  an  hour's  difference  in  the  time.     If  east  of  the 
jriven  meridian,  it  will  be  an  hour  earlier  j  if  west,  an 
Lour  later. 


reckoning  years  "f  time  ?    "What  the  difference  now  between  Old  and  New 
style,  and  why  ?    What  calendar  used  in  Russia  ? 

149.  Wliat  effect  lias  the  longitude  of  a  place  upon  its  time  ?    (Diagram, 
a.ud  explain  ?) 

150.  What  difference  of  loneritude  is  required  to  make  an  hour's  difference 
in  time?    When  earlier ?     When  later?     (How  demonstrated?    When  6 


TIME,  AS  AFFECTED  BY  LONGITUDE.  77 


1.  If  the  sun  passes  through  360°  every  24  hours,  he  must  pass  over  15®  each  hour 
as  ?<5<P  -:-  24  =:  15°.     Hence  every  15°  must  make  an  hour's  difference  in  the  time ;  and 
when  it  is  sunrise,  or  6  o'clock,  solar  time,  in  New  York  city,  it  will  be  noon,  or  12 
o'clock,  9l)°  east  of  New  York,  and  midnight  90°  west  of  it 

2.  Taking  the  circumference  of  the  earth  at  25,000  miles,  the  sun  passes  over  1041§ 
miles  every  hour  at  the  equator;  for  25.000  miles -j- 24    equals    1041§  miles.    And  if 
1041  §  miles  be  divided  by  60,  the  number  of  minutes  in  an  hour,  it  gives  about  17s  miles 
as  tiie  space  over  which  the  sun  travels  at  the  equator  every  minute.    Every  J7^  miles, 
therefore,  east  or  west,  will  make  one  minute's  difference  in  the  time.    As  we  recede 
from  the  equator  north  or  south,  the  meridians  approach  each  other,  and  a  degree  o* 
longitude  becomes  less  and  less  to  the  poles. 

2.  A  person  leaving  Boston  with  the  exact  time  will  find,  on  reaching  Albany,  about  3° 
west  of  Boston,  that  his  watch  is  some  12  minutes  ahead  of  the  Albany  time;  and  on 
reaching  Buffalo,  about  5°  further  west,  that  it  is  some  32  minutes  ahead  of  the  true  time 
at  Buffalo.  So  in  traveling  from  Buffalo  to  Boston,  the  Albany  and  Boston  time  will  be 
fou*»d  to  be  the  same  extent  ahead  of  the  Buffalo  time.  Hence  conductors  on  railroads, 
running  their  trains  by  time,  set  their  watches  from  Albany  to  Buffalo  by  some  standard 
agreed  upon— as,  for  instance,  Syracuse  time— and  reject  all  other  local  time,  bo  it  faster 
or  Blower. 

151.  As  every  15°  upon  the  earth's  surface  makes  an 
hour's  difference  in  the  time,  it  is  easy  to  convert  degrees 
into  time,  or  time  into  degrees.     By  this  means,  a  mari- 
ner having  the  time  at  the  place  whence  he  sailed,  and 
the  time  where  he   is,  from   observing  when  the  sun 
crosses  the  meridian,  can  ascertain,  from  the  difference 
between  his  standard  and  local  time,  his  distance  east  or 
west  of  the  port  whence  he  sailed,  or,  in  other  words,  his 
longitude. 

1.  Time  is  converted  into  degrees  by  multiplying  the  hours  by  15  for  the  degrees,  and 
adding  one-fourth  of  the  minutes  to  the  product;  for  every  minute  of  time  makes  \°, 
wid  every  second  of  time  -\'  in  longitude. 

2.  On  the  other  hand,  degrees  of  longitude  are  converted  into  time  by  dividing  them 
by  15  for  the  hours,  and  multiplying  the  remainder,  if  any,  by  4  for  the  minutes,  &c. 

152.  The  rotation  of  the  planets  upon  their  respective 
axes  has  caused  them  to  swell  out  at  their  equators,  and 
contract  at  their  poles — thus  assuming  the  form  of  oblate 
spheroids  (page  18). 

1.  "When  fluids  are  left  free  to  yield  to  the  influence  of  attraction,  as  mutually  existing 
between  their  particles,  they  invariably  assume  a  spherical  form.    Hence  water,  in  fall- 
ing Trom  the  clouds,  takes  the  form  of  spherical  drops;  and  melted  lead,  thrown  from 
the  top  of  a  shot-tower,  takes  a  spherical  form,  and  cooling  in  the  air  on  its  passage 
down,  remains  perfect  little  globes,  called  sJiot. 

2.  A  solid  sphere  would  never  become  oblate  by  revolution.     It  might  burst,  from 
its  powerful  centrifugal  tendency,  as  grindstones  sometimes  do  in  manufactories  of  cut- 
lery;  but  it  must  be  fluid,  or  at  least  soft  and  yielding,  in  order  to  become  oblate  by 
revolution. 

o'clock  in  New  York,  what  time  90°  east  ? — 90°  west  ?  How  many  miles  does 
the  sun  pass  over  in  an  hour  at  the  equator  ?  Per  minute  ?  How  deter- 
mined? How  north  and  south  of  equator  ?  At  45th  degree  ?  What  differ- 
ence from  Boston  to  Albany  and  Buffalo  ?  From  Buffalo  to  Boston  ?  Heneo 
what  practice  ?) 

151.  Can  time  be  converted  into  degrees,  and  decrees  into  time?    How 
useful   in  navigation?     (How  convert  time  into  degrees?     Degrees  into 
time  ?) 

152.  Effect  of  rotation   upon  figure  of  planets  ?    (Note  1  ?     2.  Solids  I 
8.  What  does  oblateuess  indicate  i    4.  Proof  from  Scripture*  I    Kemark  ?) 


78 


ASTRONOMY. 


3.  The  oblntene??  of  the  planets,  Then,  ?eems  to  indicate  tt<%  things:  Fir^t.  that  they 
were  all  once  in  a  fluid  or  plastic  state  ;  and,  secondly,  that  they  began  to  revolve  while 
in  that  state,  or  before  any  part  of  them  had  become  solid,  like  our  continents  and 
islands. 

-i.  So  far  as  the  earth  is  concerned,  we  are  taught  in  the  Holy  Scriptures — the  best 
and  most  accurate  of  all  books — that  the  earth  and  water  of  our  globe  were  once  so 
mixed,  that  the  whole  appeared  as  a  "  void"  of  "  waters;"1  and  that  they  were  afterward 
separated  into  "earth"  and  "seas"  by  the  Almighty  Creator.  (See  Genesis  i.,  2,  9,  10.) 
Thus  we  see  that  true  science  and  the  Bible  are  always  in  harmony  with  each  other. 

153.  The  difference  between  the  polar  and  equatorial 
diameters  of  the  planets,  so  far  as  known,  is  as  follows  : 
the  Earth,  26  miles ;  Mars,  25 ;  Jupiter,  6,000 ;  and 
Saturn,  7,500. 

The  oblateness  of  Jupiter  and  Saturn  is  as  plainly  visible  through  a  telescope,  as  tin 
difference  iu  the  following  figures  is  to  the  eye  of  the  student. 


ORIGINAL  FORM. 


PRESENT   APPEARANCE. 


The  plain  line  in  the  middle  figure  shows  the  original  form,  and  the  dotted  line  its 
present  form.  The  difference  is  the  change  produced  by  its  rotation.  When  measured 
by  the  proper  instruments,  it  is  found,  in  the  case  of  Jupiter,  to  amount  to  about  j'3 
of  his  average  diameter;  and  that  being  89,000  miles,  T15  is  but  little  less  than  6,000. 

154:.  As  Mercury  and  Venus  rotate  in  about  the  same 
time  of  our  globe,  and  their  Sidereal  years  are  only  88 
and  225  days  respectively  (72),  it  follows  that  Mercury 
has  but  88  natural  days  to  his  year,  and  Yenus  only 
about  225  to  hers.  But  the  natural  day  of  Jupiter  being 
only  10  hours  long,  and  his  year  equal  to  about  12  of 
ours  (11  years  317  days),  he  must  have  10,397  natural 
days  in  one  of  his  years.  So  Saturn's  year,  consisting  of 
29  years  175  days  of  our  time,  will  allow  him  to  rotate 
on  his  axis  about  25,000  times ;  or,  in  other  words,  will 
allow  of  25,000  natural  days  in  each  of  his  years.  The 
year  of  Uranus  being  equal  to  84  years  and  27  days  of 


153.  State  the  difference  of  equatorial  and  polar  diameters  of  planets? 
(Remark  respecting  Jupiter  and  Saturn  ?) 

154.  How  many  natural  days  has  Mercury  in  his  year?    Venus  ?    Jupiter? 
Row  so  many  ?    Saturn  ?    Uranus  ?    (Demonstrate.) 


TIME,    AS   AFFECTED   BY   LONGITUDE.  79 


our  time  (71),  and  his  diurnal  revolution  9J  hours  (128), 
it  follows  that  lie  has  92,683  natural  days  in  his  year. 

29  years  175  days  — 10,760  days  of  our  time;  X  24  =  258,240  hours -MOi  hours,  tho 
time  of  Saturn's  revolution.  s=  24,594^,  the  number  of  days  in  his  year.  So  84  years, 
27  days,  the  periodic  time  of  Uranus  =  36,687  days,  or  880,488  hours;  which  -=-9{  hours, 
the  time  of  the  planet's  diurnal  revolution  —  92,tiS3,  the  number  of  natural  days  in  his 
year. 

155.  As  going  from  the  earth's  center  is  to  ascend 
(page  27),  and  the  equator  of  an  oblate  spheroid  is  fur- 
ther from  the  center  than  the  poles,  it  follows,  that  the 
earth  being  an  oblate  spheroid,  we  must  ascend  some- 
what in  going  from  either  pole  to  the  equator.     A  river, 
therefore,  running  for  a  great  distance  toward  the  equator, 
would  actually  ascend /  or,  in  other  words,  run  up  hill 
— the  centrifugal  force  generated  by  the  earth's  motion 
driving  the  water  on  toward  the  equator. 

The  Mississippi  is  said  to  be  higher  at  its  mouth  than  it  is  some  thousands  of  miles 
north  of  it  If  its  bed  conforms  at  all  to  the  general  figure  of  the  earth,  this  must  cer 
tainly  be  the  case,  as  may  be  demonstrated  by  the  aid  w  .•,»,,  WTTWWT1vr<i  „„ 
ef  the  annexed  diagram.  Let  A  B  represent  the  polar,  WATEE  RUNNING  TIP  HILL. 
and  C  D  the  equatorial  diameters.  The  entire  differ- 
ence between  them  is  26  miles,  or  13  miles  on  each 
Bide.  The  two  circles  represent  this  difference.  Now 
as  the  earth's  circumference  is  25,000  miles,  the  dis- 
tance from  the  poles  to  the  equator  (being  one- 
fourth  of  that  distance)  must  be  6,250  miles;  and  in 
that  6,250  miles  the  ascent  is  13  miles,  or  over  two 
miles  to  every  1,000  toward  the  equator.  The  Mis- 
sissippi runs  from  the  50th  to  the  30th  degree  of 
north  latitude  inclusive,  or  21  degrees;  which,  at 
C9J-  miles  to  a  degree,  would  amount  to  about  1,500 
miles.  If,  then,  it  runs  a  distance  equivalent  to  1,500' 
miles  directly  south  (in  a  winding  course  of  about 
3,000),  theory  requires  that  it  should  be  about  three 
miles  higherat  its  mouth  than  it  is  1,500  miles  directly 
north  There  is  some  philosophy,  therefore,  in  saying 
that  if  a  river  runs  for  a  great  distance  from  either  pole  toward  the  equator,  it  must  run 

156.  Should  the  earth  cease  to  rotate  upon  its  axis,  the 
waters  about  the  equator  would  at  once  rush  toward  the 
poles,  flooding  them  to  the  depth  of  6^  miles,  and  reced- 
ing from  the  equator  to  the  same  amount.     So  far  as  the 
solid  portions  of  the  earth  would  permit,  it  would  at  once 
become  a  perfect  sphere.     (See  page  17,  and  also  Art.  153 
and  note.) 

157.  It  has  already  been  stated  (77),  that  the  orbits  of 
all  the  planets  were  ellipses  ;  but  they  are  not  all  alike 
eccentric.     The  orbit  of  Mercury  is  quite  elliptical,  while 

15£.  What  curious  fact  follows  from  the  earth's  oblateness?    (What  in 

g-ivcn  ?     Illustrate  by  diagram.) 
156.  What  would  be  the 'effect  should  the  earth  cease  to  rotate  ? 


so 


ASTRONOMY. 


that  of  Venus  is  nearly  a  circle.  The  student  should 
observe  that  the  eccentricity  is  not  the  deviation  from  a 
circle,  but  the  distance  from  the  center  of  an  ellipse  to 
either  focus  (see  page  23  and  cuts). 

The  eccentricity  of  the  orbits  of  the  principal  planets  is  as  follows: 


Miles. 

Ceres 21.000,000 

Pallas £4.250,000 

Jupiter 24,000,000 

Saturn 4£.OOo,o<K) 


Uranus 
Neptune. 


.85,000,000 


PRECESSION   OF  THE   EQUINOXES. 


Miles. 

Mercury 7,000.000 

Terms 492,000 

E:wth 1,618,000 

Mars 13,500,000 

Vesta 21,000,000 

Astrsea 

Juno 64,000,000 


PRECESSION  OF  THE  EQUINOXES. 

158.  The  equinoctial  points  have  already  been  defined 
(125)  as  two  points  in  the  earth's  orbit  where  the  equi- 
noctial or  celestial  equator  (20)  cuts  the  sun's   center. 
They  are  in  opposite  sides  of  the  ecliptic,  or  180°  apai't 
(see  119  and  cut).     The  vernal  equinox  is  the  point  from 
which  both  celestial  longitude  and  right  ascension  are  reck- 
oned (20  and  91) ;  but  not  being  marked  by  any  fixed  ob- 
ject in  the  heavens,  it  is  reached  just  when  the  sun  comes 
to  be  exactly  over  the  earth's  equator,  or  in  the  equinoctial 

159.  But     it    is 
found  by  long  and 
careful    observation 
that  the  earth  reaches 
the  equinoctial  point 
about     22    minutes 
and  23  seconds  ear- 
lier every  year  than 
on  the  year  preced- 
ing. This  is  equal  to 
an  arc  of  50  J"  in  the 
ecliptic.       In     this 
"manner  the  equinoc- 
tial points  are  slowly 
receding   westward, 

157.  What  said  of  the  orbits  of  Mercnry  and  Venus  ?    Of  eccentricity? 

158.  Are  the  equinoctial  points  marked  by  any  fixed  object  in  the  heavens 
How  know  when  reached  ? 

159.  Are  they  stationary  or  not  ?    Beached  how  much  earlier  armuallv 


PRECESSION     OF    THE     EQUINOXES.  81 


or  falling  back  upon  the  ecliptic,  at  the  rate  of  50J"  a 
year,  ur  1°  every  Tlf  years.  This  would  amount  to  30°, 
or  one  whole  sign  in  2,140  years,  and  to  the  entire  circle 
of  the  ecliptic  in  25,868  years. 

This  very  interesting  phenomenon  may  be  explained  by  the  preceding  diagram.  Let 
the  point  A  represent  the  vernal  equinox,  reached,  for  instance,  at  12  o'clock  on  the  2Uth 
of  March.  The  next  year  the  sun  will  be  in  the  equinoctial  22  minutes  23  seconds  ear- 
lier, at  which  time  the  earth  will  be  50i"  on  the  ecliptic,  back  of  the  point  where  the 
sun  was  in  the  equinoctial  the  year  before.  The  next  year  the  same  will  occur  again; 
and  thus  the  equinoctial  point  will  recede  westward  little  by  little,  as  shown  by  the  small 
lines  from  A  to  B,  and  from  C  to  D.  It  is  in  reference  to  the  stars  going  forward,  or 
seeming  to  precede  the  equinoxes,  that  the  phenomenon  was  called  the  Precession  of 
Iho  Equinoxes.  But  in  reference  to  the  motion  of  the  equinoxes  themselves,  it  is  rather 
a  recession. 

160.  The  cause  of  this  wonderful  motion  was  unknown, 
until  Newton  proved  that  it  was  a  necessary  consequence 
of  the  rotation  of  the  earth,  combined  with  its  elliptical 
figure,  and  the  unequal  attraction  of  the  sun  and  moon 
on  its  polar  and  equatorial  regions.     There  being  more 
matter  about  the  earth's  equator  than  at  the  poles,  the 
former  is  more  strongly  attracted  than  the  latter,  which 
causes  a  slight  gyratory  or  wabbling  motion  of  the  poles 
of  the  earth  around  those  of  the  ecliptic,  like  the  pin  of 
a  top  about  its  center  of  motion,  when  it  spins  a  little 
obliquely  to  the  base. 

161.  One  marked  effect  of  this  recession  of  the  equi 
noxes  is  an  increase  of  longitude  in  all   the  heavenly 
bodies.     As  the  vernal  equinox  is  the  zero  or  starting 
point,  if  that  recedes  westward,  it  increases  the  distance 
between  it  and  all  bodies  east  of  it ;  or,  in  other  words, 
increases  their  longitude  to  the  amount  of  its  recession. 
Hence  catalogues  of  stars,  and  maps,  showing  their  lon- 
gitude, need  to  be  corrected  at  least  every  50  years, 
otherwise  their  longitude,  as  laid  down,  will  be  too  little 
to  indicate  their  true  position.     Allowing  the  world  to 
have  stood  at  this  date  (1867)  5,871  years,  the  equinoxes 
have  receded  already  through  about  75°   of  longitude. 
At  the  same  time  the  constellations  have  gone  forward 

Hew  much  in  angular  measurement ?  Revolving?  which  way?  At  what 
rate?  Uow  long  for  1°?  For  30°  i  For  the  whole  circle  of  the  eclipti*.  ? 
(Illustrate  by  diagram.) 

160.  Cause  of  recession  ?    Who  discovered  ? 

161.  Effect  of  recession  upon  longitude?    Explain  how  effected. 
aud  constellations? 


82 


ASTRONOMY. 


eastward,  and  left  the  signs  which  bear  their  names. 
.Hence  the  sign  Aries  actually  covers  the  constellation 
Pisces. 

162.  Another  effect  of  the  recession  of  the  equinoxes 
is,  that  it  gives  to  the  pole  of  the  earth  a  corresponding 
revolution    around  the 

pole  of  the  ecliptic  in 
25,868  years. 

Let  the  line  A  A  in  the  fignire 
represent  the  plane  of  the  ecliptic; 
15  B,  the  poles  of  the  ecliptic;  CO, 
the  poles  of  the  eartli ;  and  D  1), 
the  equinoctial.  E  E  is  the  obliquity 
of  the  ecliptic.  The  star  C  at  the 
top  represents  the  pole  star,  and  the 
curve  line  passing  to  the  right  from 
it  may  represent  the  circular  orbit 
of  the  north  pole  of  the  heavens 
around  the  north  pole  of  the  ecliptic. 

163.  This     gyratory 
motion    of   the    north 
pole   of   the    heavens, 
while  it  keeps  at  the 
distance  of  23°  28'  from 

the  pole  of  the  ecliptic,  will  cause  it  to  change  its  place 
in  the  heavens  to  the  amount  of  46°  56'  in  12,934  years; 
thus  alternately  approaching  toward  and  receding  from 
the  stars,  at  every  revolution  of  the  equinoxes  around  the 
ecliptic.  Thus  the  place  of  the  pole  is  in  constant  but 
very  slow  motion  around  the  pole  of  the  ecliptic. 

164.  The  Nutation  of  the  earth's  axis  is  another  small 
and  slow  gyratory  motion,  by  which,  if  subsisting  alone, 
the  pole  would  describe  among  the  stars,  in  the  period  of 
about  19  years,  a  minute  ellipse,  having  its  longer  axis 
equal  to  18",  and  its  shorter  about  14"  ;  the  longer  axis 
pointing  toward  the  pole  of  the  ecliptic.     It  is  on  account 
of  these  varied  motions   shifting  the  point  from  which 
longitude  and  right  ascension  are  reckoned,  and  also  the 
pole  of  the  heavens,  that  it  becomes  necessary,  in  de- 

162.  What  other  effect  of  recession  ?    (Illustrate  by  diagram.) 

163.  What  elfect  npouthe  apparent  distance  of  the  stars  from  the  north 
pole  of  the  heavens  ? 

164.  What  is  Nutation?    What  meant  by  epoch,  and  why  necessary  tc 
6 cute  ? 


TELESCOPIC  VIEWS  OF  THE  PLANETS MEECUKY.          83 


scribing  the  place  of  a  star  or  planet,  by  any  of  these 
standards,  to  state  the  epoch  or  time,  and  also  whether  it 
be  mean  right  ascension — i.  e.,  right  ascension  after  hav- 
ing been  corrected  for  the  recession  of  the  equinox,  the 
zero  point. 

165.  The  Colures  are  two  great  circles  crossing  at  the 
poles  of  the  ecliptic  at  right  angles.  One  passes  through 
the  equinoxes,  and  is  thence  called  the  Equinoctial 
Colure  j  the  other  passes  through  the  solstices,  and  is 
called  the  Solstitial  Colure.  They  are  to  the  heavens 
what  four  meridians,  each  90°  apart,  would  be  to  the 
earth. 


CHAPTER   III. 

«  TELESCOPIC     VIEWS     OF     THE     PLANETS. 

166.  By  the  aid  of  telescopes,  we  discover  myriads  ot 
objects  in  the  heavens  that  are  entirely  invisible  to  the 
naked  eye  ;  while  objects  naturally  visible  are  immensely 
magnified,  and  seem  to  be  brought  much  nearer  the  ob- 
server. 

Tins  impression  of  nearness  is  an  intellectual  conclusion  drawn  from  the  fact  of  the 
increased  distinctness  of  the  object;  as  we  judge  of  the  distance  of  objects,  in  a  great 
measure,  by  their  dimness  or  distinctness. 

MERCURY. 

16T.  Under  favorable  circumstances,  Mercury  is  visible 
to  the  naked  eye,  but  yet  is  seldom  seen,  owing  to  his 
nearness  to  the  sun.  During  a  few  days  in  March  and 
April,  and  August  and  September,  he  may  be  seen  for 
several  minutes  in  the  morning  or  evening  twilight,  when 

Ifi5.  What  are  the  colures  ?    Describe. 

166.  Effect  of  the  telescope  upon  vision  ?    Upon  distant  objects  ?     (Why 
appear  nearer  ?) 

167.  Can  Mercury  be  seen  by  the  naked  eye?    Is  he  often  seen?    "Why 
not  ?    ^  hen  may  he  be  seen  ?    How  appear  ? 


84  ASTRONOMY. 


lii»  greatest  elongations  (99)  happen  in  those  months 
lie  appears  like  a  star  of  the  third  magnitude,  with  a 
pale  rosy  light.  See  104  and  note. 

168.  Through  a  telescope,  Mercury  exhibits  different 
phases  in  different  parts  of  his  orbit,  similar  to  those  pre- 
sented by  the  moon  in  her  revolution  around  the  earth. 
The  German  astronomer,  Schroeter,  discovered  numerous 
mountains  upon  the  surface  of  Mercury,  one  of  which  he 
estimated  to  be  nearly  11  miles  in  hight.     By  observing 
these  at  different  times,  he  determined  the  diurnal  revo- 
lution of  the  planet  to  be  24h.  5m.  28s.   But  these  observa- 
tions have  riot  been  confirmed  by  any  other  astronomer. 
The  apparent  angular  diameter  of  Mercury  varies  from 
5"  to  12",  according  to  his  position  with  respect  to  the 
earth  (56  and  80).     So  far  as  is  known, he  is  not  attended 
by  any  satellite. 

VENUS. 

169.  When  favorably  situated,  Yenus  is  one  of  the 
most  conspicuous  members  of  the  planetary  system,  and 
is  a  most  brilliant  object  even  to  the  naked  eye.     Her 
color  is  of  a  silvery  white,  and,  when  at  a  distance  from 
the  sun,  either  east  or  west,  she  is  exceedingly  bright  and 
beautiful.     When  nearest  the  earth,  her  apparent  di- 
ameter is  61",  which  is  greater  than  that  of  any  other 
planet,  owing  to  her  being  so  much  nearer  than  Jupiter 
or  Saturn. 

Under  a  telescope,  Yenus  exhibits  all  the  phases  of 
the  moon,  as  she  revolves  around  the  sun.  The  cause  of 
this  phenomenon  is,  that  we  see  more  of  her  enlightened 
side  at  one  time  than  at  another ;  and  the  same  is  true 
of  Mercury. 

1.  The  telescopic  nppenr  moo  of  Venus,  at  different  points  in  her  orbit,  is  represented 
iu  the  following  iigure.    At  E  and  W  she  has  her  greatest  eastern  and  western  elon»»- 

168.  How  appear  through  telescope?     What  said  of  Schroeter  ?    "What 
conclusion  from  observing  the  spots?    Confirmed  by  others,  or  not$    An- 
gular diameter  of  Mercury  ?     Why  vary  ?    Has  he  a  satellite  ? 

169.  What  said  of  Venus  I    Her  apparent  diameter?    Why  greater  than 
that  of  Jupiter  ?    How  appear  through  telescope  ?     Cause  of  her  phases  ? 
(Describe  phases  when  east  of  the  sun — west.     What  prediction  befo** 
the  discovery  of  the  telescope  ?) 


TELESCOPIC  VIEWS  OF  VENUS. 


85 


tion,  and  is  stationary;  while  her  positions  oppoMts  the  words  " direct"  and  "retro- 
grade" represent  her  at  her  conjunctions.  The  spots  on  the  face  of  the  sun  represent 
Venus  projected  upon  his  disk,  in  a  transit,  the  arrow  indicating  her  direction. 


TFT.F.ROOP10    PHASES    OF    VENUS. 


2.  Before  the  discovery  of  the  telescope,  it  was  asserted  that  if  the  Copernican  theory 
were  true.  Mercury  and 'Venus  would  exhibit  different  phases  at  different  times:  and  as 
those  phases  could*  not  he  seen,  it  was  evident  that  the  theory  was  false.  But  no  sooner 
had  Galileo  directed  his  small  telescopes  to  these  objects,  than  he  found  them  exhibiting 
the  very  appearances  required  by  the  Copernican  theory,  its  opponents  themselves  being 
judges. 

170.  Besides  the  phases  above  mentioned,  a  close  in- 
spection of  Yenus  will  reveal  a  variety  of  spots  upon 
her  surface.  These  are  supposed  to  be  the  natural  divi- 
sions of  her  surface,  as  continents,  islands,  &c.  Schroeter 
measured  several  mountains  upon  this  planet,  one  of 
which  he  estimated  at  over  twenty  miles  in  bight.  There 
is  evidence  of  the  existence  of  an  atmosphere  about  this 
planet,  extending  to  the  distance  of  about  three  miles. 


BPOTS  SEEN  UPON  THE  SURFACE  OP  YENUS. 


171.  "Were  a  person  situated  upon  one  of  the  exterioi 
planets,  at  a  distance  from  our  globe,  it  would  exhibit 
phases  like  Mercury  and  Venus,  in  its  annual  revolution  ; 
and  the  continents,  islands,  and  seas  would  appear  only 
as  spots  upon  her  surface,  assuming  various  forms,  ac- 
cording to  the  position  from  which  they  were  viewed. 

170.  What  else  seeu  upon.  Venus  ?    What  supposed  to  be  ?    Schroeter's 
measurements  ?    Has  Venus  an  otmoftphtrtf 

171.  How  would  our  globe  appear  if  viewed  from  a  dLstauoe  ? 


ASTRONOMY. 


DISTANT   TELESCOPIC   VIEWS   OF   THE   EARTH. 
2.  3. 


Above  we  have  four  different  views  of  our  own  globe.  No.  1  is  a  view  of  th« 
N  w-thern  Hemisphere ;  No.  2,  of  the  Southern  ;  No.  3,  of  the  Eastern  Continent ;  No.  4^ 
of  the  Western.  A  common  terrestrial  globe  will  present  a  different  aspect  from  every 
new  position  from  which  it  is  viewed ;  as  the  earth  must  in  her  appearance  to  the  in- 
habitants of  other  worlds. 

MARS. 

172.  Mars  usually  appears  like  a  star  of  the  second 
magnitude,  of  a  reddish  hue.     When  in  opposition,  or 
nearest  to  the  earth,  he  appears  quite  brilliant,  as  we  see 
his  disk  fully  illuminated.     His  apparent  diameter  is 
then  about  18" ;  whereas,  when  on  the  opposite  side  of 
the  ecliptic,  or  in  conjunction  with  the  sun  (80),  it  is  only 
4".     He  exhibits  slight  phases,  and  his  surface  seems  to 
be  variegated  with  hill  and  vale,  like  the  other  planetary 
bodies.     "  Upon  this  planet,"  says  Dr.  Herschel,  "  we 
discern,  with  perfect  distinctness,  the  outlines  of  what 
may  be  continents  and  seas."     When  it  is  winter  at  his 
7iorth  pole,  that  part  of  the  planet  is  white,  as  if  covered 
with  ice  and  snow  ;  but  as  summer  returns  to  his  north- 
ern hemisphere,  the  brightness  about  his  north  pole  dis- 
appears. 

173.  The  general  ruddy  color  of  Mars  is  supposed  by 
Sir  John  Herschel  to  indicate  "  an  ochery  tinge  in  the 
general  soil,  like  what  the  red  sandstone  districts  on  the 
earth  may  possibly  offer  to  the  inhabitants  of  Mars." 
Others  suppose  it  to  indicate   the  existence  of  a  very 
dense  atmosphere,  which  analyzes  the  light  reflected  from 
the  planet. 

When  the  sunlight  passes  through  vapor  or  clouds  in  the  morning  or  evening,  the 
different  rays  of  which  it  is  composed  are  separated,  and  the  red  rays  only  pass  to  the 

172.  Usual  appearance  of  Mars  ?  When  brightest,  and  why  ?  Apparent 
d  unreter  ?  Cause  of  great  variation  ?  Phases  ?  Herschel's  remark  ?  Spo*. 
a  L  rth  poie  ? 

lid.  Supposed  causes  of  his  color  f    (Note.) 


THE    ASTEROIDS. 


87 


earth,  giving  to  the  clouds  a  gorgeous  crimson  appearance.    In  a  similar  manner  it  ia 
supposed  that  the  atmosphere  of  Mars  may  give  him  his  crimson  hue. 


TELESCOPIC   APPEARANCES   OF   MAES. 


1.  The  right-hand  figure  represents  M;irs  as  seen  at  the  Cincinnati  Observatory, 
August  5, 1S45.     On  the 80th  of  tlie  same  month  he  appeared  as  represented  on  the  left. 
The  middle  mew  is  from  a  drawing  by  Dr.  Dick. 

2.  Just  east  of  the  '•  Seven  Stars,"  or  Pleiade*,  the  student  will  find  another  group 
sailed  the  Jfyades  ;  one  of  which,  called  Aldebaran,  is  of  a  reddish  cast,  and  somewhat 
resembles  the  planet  Mars.     When  Mars  is  in  opposition,  however,  at  his  nearest  point 
tons,  and  with  his  enlightened  side  toward  us,  he  appears  much  larger  and  brighter  than 
Aldebaran. 

174:.  As  the  periodic  time  of  Mars  is  only  1  yr.  322 
days  (71),  his  motion  eastward  among  the  stars  will  be 
very  rapid,  as  in  that  time  he  must  traverse  the  whole 
circle  of  the  heavens.  His  rate  of  motion  being  about 
1°  for  every  two  days,  or  one  whole  sign  in  57  days,  it 
will  be  easy  to  detect  his  eastward  progress  by  observing 
his  change  of  position  with  reference  to  the  fixed  stars, 
for  a  few  evenings  only  ;  and  by  marking  his  place  occa- 
sionally for  two  years,  we  may  track  him  quite  around 
the  heavens. 

THE    ASTEKOIDS. 

175.  The  Asteroids  are  invisible  except  through  tele- 
scopes, though  Yesta  was  once  seen  by  Schroeter  with 
the  naked  eye.  Few  of  them  present  any  sensible  disks, 
even  under  the  telescope.  They  have  a  pale  ash-color, 
with  the  exception  of  Ceres,  which  is  of  a  reddish  hue, 
resembling  Mars.  A  thin  haze  or  nebulous  envelope  has 
been  observed  around  Pallas,  supposed  to  indicate  an 
extensive  atmosphere  ;  but  no  spots  or  other  phenomena 
have  ever  been  detected. 

"On  such  planets,"  says  Sir  John  Ilerschel,  "giants  might  exist;  and  those  animals 
which  on  earth  require  the  buoyant  power  of  water  to  counteract  their  weight  might 

174.  What  said  of  the  eastward  motion  of  Mars?    How  detected  ?    Rate  ? 

175.  Are  the  asteroids  visible  to  naked  eye  ?    Schroeter  ?    How  appear 
•ander  telescope  I    Ceres  ?    Pallas  I    (Remarks  of  Sir  John  Ilerschel  'i} 


88 


ASTRONOMY. 


TELESCOPIC  VIEW  OF  JUPITER. 


there  be  denizens  of  the  land.  A  man  placed  on  one  of  these  planets  might  pprinsr  with 
ease  to  the  hight  of  60  feet,  and  sustain  no  greater  shock,  in  his  descent  than  he  does  on 
the  earth  from  leaping  a  yard."  See  05  to  67,  and  notes. 

JUPITER. 

176.  To  the  naked  eye,  Jupiter  appears  like  a  fine 
bright  star  of  the  first  magnitude.  His  apparent  di- 
ameter varies  from  30"  to  46",  according  to  his  distance 
from  the  earth.  His  color  is  of  a  pale  yellow.  Under  a 
telescope,  his  ob- 
lateness  is  plainly 
perceptible  (as 
shown  at  135), 
and  his  disk  is 
seen  to  be  streak- 
ed with  curious 
belts,  running 
parallel  to  his 
equator,  as  shown 
in  the  cut. 

1.  The  number  of  belts 
to  be  seen  upon  the  disk 
of  Jupiter  depends  very 
much  upon  the  power  of 
the  instrument  through 
which  he  is  viewed.  An 
ordinary  telescope  will 

show  the  two  main  belts,  one  each  side  of  his  equator;  but  those  of  greater  power  ey- 
hibitmore  of  these  curious  appendages.  Dr.  Herschel  once  saw  his  whole  disk  covered 
with  small  belts. 

ITT.  These  belts  sometimes  continue  without  change 
for  months,  and  at  other  times  break  up  and  change  their 
forms  in  a  few  hours.  They  are  quite  irregular,  both  in 
form  and  apparent  density  /  as  both  bright  and  dark 
spots  appear  in  them,  and  their  edges  are  always  broken 
and  uneven.  They  are  supposed  by  some  to  be  openings 
in  the  atmosphere  of  the  planet,  through  which  its  real 
body  is  seen;  while  others  think  they  may  be  clouds, 
thrown  into  parallel  strata  by  the  rapid  motion  of  Jupi- 
ter upon  his  axis.  The  S2}ots  in  the  belts  are  thought  to 

176.  Jupiter  to  naked  eye?    Apparent  mnrrnitude?    Cause  of  variation  ? 
Color?     Figure?     Belts?"    (Number  of  belts  ?     Ordinarily?     As  seen  by 
Herschel  ?     What  view  in  the  cut  ?) 

177.  Are  these  belts  permanent  and  regvlar?    "What  supposed  to  be? 
What  said  of  spots  in  the  belts?    What  ascertained  by  observing  spots1 
( What  said  in  note  ?) 


SATUKS".  89 


be  caverns  or  mountains,  or,  at  least,  something  perma 
nently  attached  to  the  body  of  the  planet.  It  was  by 
watching  these  that  the  rotation  of  the  planet  upon  his 
axis  was  ascertained. 

One  of  these  spots,  first  observed  in  1665,  disappeared,  and  reappeared  regularly  in  the 
same  form  for  more  than  forty  years;  showing  conclusively  that  it  was  something  per 
manent,  and  not  a  mere  atmospherical  phenomenon. 

178.  In  examining  Jupiter  with  a  telescope,  from  one 
to  four  small  stars  will  be  seen  near  him,  which,  on 
examination,  will  be  found  to   accompany  him  in  his 
eastward  journey  around  the  heavens,  and  to  revolve 
statedly  around  him.     These  are  the  moons  of  Jupiter, 
of  which  we  shall  speak  more  fully  under  the  head  of 
Secondary  Planets. 

The  writer  once  saw  all  four  of  these  satellites  at  once,  and  very  distinctly,  through 
a  common  ship  telescope,  worth  only  twelve  or  fifteen  dollars.  They  were  first  seen 
by  Galileo  with  a  telescope,  the  object-glass  of  which  was  only  one  inch  in  diameter! 
If  the  student  can  get  hold  of  any  such  instrument  whatever,  let  him  try  it  upon  Jupi- 
ter, and  see  if  he  cannot  find  from  one  to  four  small  stars  near  him,  that  will  occupy 
dilferent  positions  at  dift'erent  tiinea. 

179.  As  the  periodic  time  of  Jupiter  is  11  years  317 
days  (71),  his  rate  of  motion  eastward  through  the  fixed 
stars  is  about  30°  a  year.     Still,  this  motion  can  soon  be 
detected,  and  in  12  years  we  may  watch  his  progress 
quite  around  the  heavens. 

The  writer  has  -watched  this  planet  frojn  the  constellation  of  Aries,  west  of  the 
»•  Seven  Stars,"  till  he  passed  that  group,  and  onward'  through  s ,  n,  ®,  &c.,  to  his  for- 
mer poftition.  He  then  commenced  his  second  round  under  our  observation,  which 
he  completed  in  1860,  and  now  (July,  1866)  has  completed  over  one-half  of  his  third 
journey  of  the  celestial  circla,  which  "he  performs  regul&rly  every  twelve  years. 

SATURN. 

180.  This  planet  is  plainly  visible  to  the  naked  eye, 
appearing  like  a  star  of  the  third  magnitude,  of  a  pale 
bluish  tint.     His  average  angular  diameter  is  about  18". 
By  the  aid  of  the  telescope,  he  is  found  not  only  to  be 
oblate,  and  striped  with  belts,  and  attended  by  satellites 
like  Jupiter,  but  to  be  encircled  by  a  suite  of  gorgeous 
rings,  which  renders  him  one  of  the  most  interesting 
objects  in  all  the  heavens. 

178.  What  else  discovered  about  Jupiter  ?    What  are  they  ?    (Kemark  in 
note  ?) 

179.  Jupiter's  rate  of  motion  eastward?    Is  it  easily  detected  ?    (Kemark 
in  note  ?    Where  was  Jupiter  in  1846  ?    In  1853  ?    Where  now  ?    Where  in 
1858  ?) 

180.  Natural  appearance  of  Saturn?     Angular  diameter?     Appearance 
through  telescope  4 


90  ASTRONOMY. 


181.  The  oblateness  of  Saturn  (15)  is  distinctly  visi 
ble  through  good  telescopes  (as  shown  in  the  cut),  while 
the  body  of  the  planet  is  of  a  lead  color,  and  the  rings 
of  a  ttilvery  white.     They  may  be  compared  to  concen- 
tric  circles  (18)  cut  out  of  a  sheet  of  tin.     They  are 
broad,  flat,  and  thin,  and  are  placed  one  within  the  other 
directly  over  the  equator  of  the  planet,  and  revolve  with 
him  about  his  axis,  in  the  same  direction,  and  in  the 
same  time  (128).     They  are  estimated  to  be  about  100 
miles  in  thickness. 

182.  These  rings  are  solid  matter,  like  the  body  of  the 
planet.     This  is  proved  by  the  fact  that  they  invariably 
cast  a  strong  shadow  upon  the  body  of  the  planet,  and 
frequently  exhibit  the  planet's  shadow  very  distinctly 
upon  their  own  surfaces.     It  is  also  evident  that  they 
are  wholly  detached  from  the  planet,  as  the  fixed  stars 
in  the  distant  heavens  beyond  have  been  seen  through 
the  opening  in  the  rings,  and  between  the  planet  and 
the  first  ring. 

TELESCOPIC  TIKW  OF  SATURN. 

The  adjoining  cut  is  an  ex- 
cellent representation  of  Sa- 
turn as  seen  through  a  tele- 
scope. The  oblateness  of  the 
planet  is  easily  perceptible, 
and  his  »h<idow  can  be  seen 
upon  the  rings  back  of  the 
planet  The  shadow  of  the 
rings  may  also  be  seen  run- 
ning across  his  disk.  The 
writer  has  often  seen  the 
opening  between  the  body  of 
the  planet  and  the  interior 
ring  as  distinctly  as  it  appears 
to  the  student  in  the  cut.  Un- 
der very  powerful  telescopes, 
these  rings  are  found  to  be  again  subdivided  into  an  indefinite  number  of  concentric 
elides,  one  within  the  other,  though  this  is  considered  doubtful  by  Sir  John  IIcrscheL 

183.  As  our  view  of  the  rings  of  Saturn  is  generally 
an  oblique    one,    they    usually    appear   elliptical,   and 
never  circular.     The  ellipse  seems  to  contract  for  about 
7^  years,  till  it  almost  entirely  disappears,  when  it  begins 

181.  Oblateness?     Color?     Kings— what  like?     How  situated?     What 
motion  ?    Thickness  ? 

182.  What  said  of  substance  of  the  rings  ?    What  proof?    What  evidence 
that  they  are  detached  ?    (Remark  of  author  as  to  seeing  satellites  ?    lle- 
specting  rings  ?     Opinion  of  Ilcrschel  ?) 

183.  What  the  general  apparent  figure  of  the  rings  ?    Why  elliptical  1 


SATUEN. 


91 


to  expand  again,  and  continues  to  enlarge  for  T-J  years, 
when  it  reaches  its  maximum  of  expansion,  and  again 
begins  to  contract.  For  fifteen  years,  the  part  of  the 
rings  toward  us  seems  to  be  thrown  up,  while  for  the 
next  fifteen  it  appears  to  drop  below  the  apparent  center 
of  the  planet ;  and  while  shifting  from  one  extreme  to 
the  other,  the  rings  become  almost  invisible,  appearing 
only  as  a  faint  line  of  light  running  from  the  planet  in 
opposite  directions.  The  rings  vary  also  in  their  inclina- 
tion, sometimes  dipping  to  the  right,  and  at  others  to  the 
left, 

TKLESCOPIO   PHASES   OF  THE   KINGS   OF  SATURN. 


The  above  is  a  good  representation  of  the  various  inclinations  and  degrees  of  expan- 
sion of  the  rings  of  Saturn,  during  his  periodic  journey  of  30  years. 

184.  The  rings  of  the  planet  are  always  directed  more 

Or  leSS  toward  the  earth,      PERPENDICULAR  VIEW  OF  THE  RINGS  OF  SATURN. 

and  sometimes  exactly 
toward  us ;  so  that  we 
never  see  them  perpen- 
dicularly, but  always 
either  exactly  edge- 
wise, or  obliquely,  as 
shown  in  the  last  figure. 
Were  either  pole  of  the 
planet  exactly  toward 
us,  we  should  then  have 
a  perpendicular  view  of 
the  rings,  as  shown  in 
the  adjoining  cut. 

185.  The  various   phases    of  Saturn's   rings   are   ex- 
plained by  the  facts  that  his  axis  remains  parallel  to  it- 
self (see  following  cut),  with  a  uniform  inclination  to  the 

What  periodic  variation  of  expansion  ?    Of  inclination  ?     When  nearly  in- 
visible { 

184.  How  are  the  rings  situated  with  respect  to  the  earth.  ?    How  wouH 
taey  appear  if  either  pole  of  Saturn  were  toward  us  ? 


92 


ASTRONOMY. 


plane  of  his  orbit  (122),  which  is  very  near  the  ecliptic 
(108) ;  and  as  the  rings  revolve  over  his  equator,  and  at 
right  angles  with  his  axis,  they  also  remain  parallel  to 
themselves.  The  revolution  of  the  planet  about  the  earth 
every  30  years  (72)  must  therefore  bring  first  one  side 
of  the  rings  to  view,  and  then  the  other — causing  all  the 
variations  of  expansion,  position,  and  inclination  which 
the  rings  present. 

SATURN   AT  DIFFERENT  POINTS  IN  HIS   ORBIT. 


1.  Here  observe,  first,  that  the  axis  of  Saturn,  like  those  of  all  the  other  planets, 
remains  permanent,  or  parallel  with  itself;  and  as  the  rings  are  in  the  plane  of  his 
equator,  and  at  right  angles  with  his  axis,  they  also  must  remain  parallel  to  themselves, 
whatever  position" the  planet  may  occnpj'  in  its  orbit. 

2.  This  being  the  case,  it  is  obvious  that  while  the  planet  is  passing  from  A  to  E,  the 
6vm  will  shine  upon  the  under  or  south  side  of  the  rings;  and  while  he  passes  from  E 
to  A  again,  upon  the  upper  or  north  side ;  and  as  it  requires  about  30  years  for  the 
planet  to  traverse  these  two  semicircles,  it  is  plain  that  the  alternate  day  and  night  on 
the  rings  will  be  15  years  each. 

8.  A  and  E  are  the  equinoctial,  and  C  and  G  the  solstitial  points  in  the  orbit  of 
Saturn.  At  A  andE  the  rings  are  edgewise  toward  the  sun,  and  also  toward  the  earth, 
provided  Saturn  is  in  opposition  to  the  sun.  To  an  observer  on  the  earth,  the  rings  will 
seem  to  expand  from  A  to  C,  and  to  contract  from  C  to  E.  So,  also,  from  E  to  G,  and 
'from  G  to  A.  Again :  from  A  to  E  the  front  of  the  rings  will  appear  above  the  planet's 
center,  and  from  'E  to  A  lelow  it. 

4.  The  rings  of  Saturn  were  in  visible,  as  rings,  from  the  22d  of  April,  1848,  to  the  19th 
of  January,  1849.     He  came  to  his  equinox  September  7,1848;  from  which  time  to 
February,  1856,  his  rings  will  continue  to  expand.    From  that  time  to  June,  1863,  they 
will  contract,  when  he  will  reach  his  other  equinox  at  E,  and  the  rings  will  be  invisible. 
From  June,  1863,  to  September.  1870,  they  will  again  expand;  and  from  September, 
1870,  to  March,  1877,  they  will  contract,  when  he  will  be  at  the  equinox  passed  Septem- 
ber 7, 1848,  or  29i  years  before. 

5.  The  writer  has  often  seen  the  rings  of  Saturn  in  different  stages  of  expansion  and 
contraction,  and  once  when  they  were  almost  directly  edgewise  toward  the  earth.     At 
that  time  (January,  1849),  they  appeared  as  a  bright  line  of  light,  as  represented  at  A 
and  E,  in  the  above  cut. 

185.  What  is  the  cause  of  these  varying  phases,  &c.  ?  (Explain  by  dia- 
gram. When  rings  invisible?  When  at  his  equinox  ?  How  long  rings  ex- 
pand ?  Contract?  When  rings  next  invisible?  Expansion  again?  Con- 
traction ?  At  what  point  then  ?  Author's  observations  ?) 


SATURN.  £3 


1S6.  The  dimensions  of  the  rings  of  Saturn  may  bo 
stated  in  round  numbers  s$  follows  : 

Miles. 

Distance  from  the  body  of  the  planet  to  the 

first  ring  . 19,000 

Width  of  interior  ring 17,000 

Space  between  the  interior  and  exterior  rings    2,000 

Width  of  exterior  ring 10,500 

Thickness  of  the  rings 100 

These  statistics,  as  given  by  Sir  John  Herschel,  are  as  follows : 

Exterior  diameter  of  exterior  ring.  • 40"'095  =r  176.41S  miles. 

Interior  do ! 35" -289  =  155.272  " 

Exterior  diameter  of  interior  ring 34" -475  =  151. (>90  " 

Interior  do 26"-6(>S  =  117,339  " 

Equatorial  diameter  of  the  body 17" -991  =    79,160  " 

Interval  between  the  planet  and  interior  ring 4" -339  =    19.090  " 

Interval  of  the  rings 0"'408  =     1,791  " 

Thickness  of  the  rings  not  exceeding . ...  250  " 

187.  The  rings  of  Saturn  serve  as  reflectors  to  reflect 
the  light  of  the  sun  upon, his  disk,  as  our  moon  reflects 
the  light  to  the  earth.     In  his  nocturnal  sky,  they  must 
appear  like  two  gorgeous  arches  of  light,  bright  as  the 
full  moon,  and  spanning  the 

whole  heavens  like  a  stupen- 
dous rainbow. 

In  the  annexed  cut,  the  beholder  is  supposed 
to  be  situated  some  30°  north  of  the  equator  of 
Saturn,  and  looking  directly  south.  The  shad- 
ow of  the  planet  is  seen  travelling  up  the  arch 
as  the  night  advances,  while  a  New  Moon  is 
shown  in  the  west,  and  &FuftA[oon  in  the  east 
at  the  same  time. 

188.  The  two  rings  united  are  nearly  13  times  as  wide 
as  the  diameter  of  the  moon ;  and  the  nearest  is  only 
T^th  as  far  from  the  planet  as  the  moon  is  from  us. 

1.  The  two  rings  united  are  27,500  miles  wide;  which -f- 2, 160  the  moon's  diame- 
ter — 12^.    So  240,00u  miles,  the  moon's  distance  -7- 19,000  the  distance  of  Saturn's  in- 
terior ring  =  12|-f. 

2.  At  the  distance  of  only  19,000  miles,  our  moon  would  appear  some  forty  times  as 
large  as  she  does  at  her  present  distance.     How  magnificent  and  inconceivably  grand, 
then,  must  these  vast  rings  appear,  with  a  thousand  times  the  moon's  magnitude,  and 
only  one-twelfth  part  of  her  distance  1 

186.  State  the  distances  and  dimensions  of  his  rings,  beginning  at  the  body 
•of  the  planet,  and  passing  outward.    (What  additional  statistics  from  Her- 
bchel?) 

187.  What  purpose  do  the  rings  of  Saturn  serve?    How  appear  in  his 
evening  sky  ? 

188.  Width  of  two  rings,  as  compared  with  moon  ?    Distance  ?    (Demon- 
strate both.    How  would  our  moon  appear  at  the  dist-ince  of  Saturn's  rings  1) 


NIGHT  SCENE   UPON   SATURN. 


94  ASTRONOMY. 


189.  Besides  the  magnificent  rings  already  described, 
the  telescope  reveals  eight  satellites  or  moons,  revolving 
around  Saturn.     But  these  are  seen  only  with  good  in- 
struments, and  under  favorable  circumstances. 

On  one  occasion,  the  writer  saw  five  of  them  at  once,  with  a  six-inch  refractor  manu- 
factured by  Mr.  Henry  Fitz,  of  New  York  ;  but  the  remaining  three  lie  has  never  seen. 
For  a  further  description  of  these  satellites,  see  chapters  on  the  Secondary  Planets. 

190.  The  periodic  time  of  Saturn   being   nearly  30 
years  (72),  his  motion  eastward  among  the  stars  must  be 
very  slow,  amounting  to  only  12°  a  year,  or  one  sign  in 
2^  years.     It  will  be  easy,  therefore,  having  once  ascer- 
tained bis  position,  to  watch  his  slow  progress  eastward 
year  after  year.     Saturn  is  now  (October,  1852)  about 
15°  west  of  the  seven  stars,  and  consequently  will  pass 
them  eastward  early  in  1854. 

UKANUS. 

191.  Uranus  is  scarcely  ever  visible  except  through  a 
telescope ;  and  even  then  we  see  nothing  but  a  small 
round  uniformly  illuminated  disk,  without  rings,  belts, 
or  discernible  spots.      His  apparent  diameter  is  about 
4",  from  which  he  never  varies  much,  owing  to  the  small- 
ness  of  our  orbit  in  comparison  with  his  own. 

Sir  John  Herschel  says  he  is  without  discernible  spots,  and  jret  in  his  tables  he  lays 
down  the  time  of  the  planet's  rotation  (which  could  only  be  ascertained  by  the  rotation 
of  spots  upon  the  planet's  disk),  at  9i  hours  (1'28).  This  time  is  probably  given  on  the 
authority  of  Bchroeter,  and  is  marked  as  doubtful  by  Dr.  Herschel. 

192.  The  motion  of  Uranus  in  longitude  is  still  slower 
than  that  of  Saturn.     His  periodic  time  being  84  years 
27  days,  his  eastward  motion  can  amount  to  only  about 
4°  17'  in  a  whole  year.     To  detect  this  motion  requires 
instruments  and  close  observations.     At  this  date  (1853) 
Uranus  has  passed  over  about  -8  of  his  orbit,  since  his 
discovery  in  1781  ;  and  in  1865  will  have  traversed  the 
whole  circuit  of  the  heavens,  and  reached  the  point 
where  Herschel  found  him  84  years  before. 

189.  What  else  seen  about  Saturn?    When  seen?    (Observations  of  tho 
author.) 

190.  Motion  of  Saturn  eastward  ?    Rate  ? 

191.  How  Uranus  seen?     How  appear  through  telescopes?    Apparent 
diameter  ?    Why  so  small,  when  so  much  larger  than  Venus  (    Why  so  little 
variation  ?    (Remark  respecting  spots.) 

li)2.  What  said  of  Uranus'  apparent  motion?  Rate  per  year?  In  1853 
how  far  since  discovered  2  When  made  a  complete  revolution  since  1781  ? 


THE    SOLAR   SYSTEM   IN   MINIATURE.  05 


193.  Uranus  is  attended  by  several  satellites — four  at 
least,  probably  five  or  six. 

Sif  William  Herschel  reckoned  six,  though  no  other  observer  has  confirmed  this 
epinion ;  and  even  his  son,  Sir  John  Herschel,  seems  to  consider  the  existence  of  six 
satellites  quite  doubtful. 

NEPTUNE. 

194.  Neptune  is  a  purely  telescopic  planet,  and  his 
immense  distance  seems  to   preclude  all  hope  of  our 
coming  at  much  knowledge  of  his  physical  state.     A 
single  satellite  has  been  discovered  in  attendance  upon 
him,  and  the  existence  of  another  is  suspected ;  but  ii 
others  exist,  they  are  as  yet  undetected. 

195.  On  the   3d  of  October,  1846,  Mr.  Lassell,  cf 
Liverpool,  England,  supposed  he  had  discovered  a  ring 
about  the  planet,  similar  to  the  rings  of  Saturn  ;  but  this 
supposition  has  not  yet  been  confirmed  by  th^  observa- 
tions of  other  astronomers. 

196.  The  periodic  time  of  Neptune  being  164  years 
226  days,  his  motion  in  longitude  amounts  to  only  about 
2°  10'  per  year;  and  yet  this  slow  motion  of  about  21" 
per  day  is  easily  detected,  in  a  short  time,  by  the  rid  of 
the  proper  instruments.     It  is  by  this  motion,  as  w^ll  as 
by  the  disk  which  it  exhibits  under  the  telescope,  that 
the  object  was  first  distinguished  from  the  fixed  stars, 
and  recognized  as  a  planet. 

THE   SOLAR   SYSTEM   IN   MINIATURE. 

197.  Choose  any  level  field  or  howling-green,  and  in 
its  center  place  a  globe  two  feet  in  diameter,  to  represent 
the  sun.     Mercury  may  then  be  represented  by  a  mus- 
tard-seed, at  the  distance  of  82  feet ;   Venus  by  &  pea,  at 
the  distance  of  142  feet ;  the  earth  aluo  by  a  pea,  at  the 
distance  of  215  feet.     A  large  pin's  head  would  repre- 
sent Mars,  if  placed  327  feet  distant;  svod  the  A.steroids 
may  be  represented  by  grains  of  sand.  fr?ra  500  to  600 

193.  Attendants  of  Uranus  ?    How  many  ?    (Remark  .n  note  !) 

194.  How  Neptune  seen?     What  attendant  ?    SUS-PIUKU  t 
J  9n.  Supposition  of  Lassell  ?    Is  it  confirmed  ? 

196.  Motion  of  Neptune  per  year  '{     Why  so  slow  ?    ^••»»  't  hp  detected  » 

197.  What  representation  of' the  solar  system?    Size  Oi'  doo  1    Merc!iry. 


96  ASTRONOMY. 


feet  from  the  center.  A  moderate  sized  orange,  would  rep- 
resent Jupiter,  at  the  distance  of  80  rods,  or  1,320  feet ; 
while  a  smaller  orange  would  represent  Saturn,  at  the 
distance  of  124  rods,  or  2,046  feet.  Place  a  full-sized 
cherry  or  small  plum  three-fourths  of  a  mile  distant  for 
Uranus,  and  another  a  mile  and  a  quarter  distant  for 
Neptune,  and  you  have  the  solar  system  in  miniature. 

198.  To  imitate  the  motions  of  the  planets  in  their 
orbits,  in  the  above  illustration,  Mercury  must  move  to 
the  amount  of  his  own  diameter  in  41  seconds ;  Yen  us, 
in  4m.  14s. ;  the  earth,  in  7m. ;  Mars,  in  4m.  48s. ;  Jupi- 
ter, in  2h.  56m. ;  Saturn,  in  3h.  13m. ;  Uranus,  in  2h. 
16m. ;  and  Neptune,  in  3h.  30m. 


CHAPTER    IV. 

SEASONS  OF  THE  DIFFERENT  PLANETS,  ETC. 

199.  The  general  philosophy  of  the  seasons  has  already 
been  explained  (Art.  119  to  125). 

The  inclination  of  the  axis  of  a  planet  determines  the 
extent  and  character  of  its  zones  /  and  the  length  of  its 
periodic  time  determines  the  length  of  its  seasons. 

Thus  the  axis  of  the  earth  being  inclined  toward  the  ecliptic  2-3°  28',  the  tropics  fall 
23°  28'  from  the  equator,  and  the  polar  circles  23°  2S'  from  the  poles;  and  the  period  of 
the  earth's  revolution  around  the  sun  being  365^  days,  it  follows  that  each  of  the  lour 
seasons  must  include  about  three  months,  or  91  days  on  an  average.  It  the  axis  were 
wore  inclined,  the  tropics  would  full  further  from  the  equator,  and  the  polar  circles  fur- 
ther from  the  poles,  so  that  the  torrid  and  frigid  zones  would  be  wider,  and  the  tem- 
perate narrower;  and  if  the  earth's  period  were  longer,  her  seasons,  respectively,  would 
be  longer. 

200.  The  general  temperature  of  a  planet  is  probably 
governed  by  its  distance  from  the  sun  (59,  60) ;  but  the 
temperature  of  any  particular  portion  of  a  planet  de- 
pends mainly  upon  the  directness  or  obliquity  with  which 

and  where  placed  I    Venus,  what  and  where  ?    Earth  ?    Asteroids  ?    Mars 
&c. 

198.  How  imitate  the  motions  of  the  several  planets  ? 

199.  What  determines  the  extent  and  character  of  a  planet's  zones  ?    What 
tLe  length  of  its  seasoua  ?    (Illustrate  by  inclination  and  period  of  the  earth.) 


SEASONS   OF   THE   DIFFERENT   PLANETS,   ETC.  97 

the  rays  of  light  fall  upon  it — a  circumstance  that  greatly 
affects  the  amount  of  light  received  by  any  given  por- 
tion of  its  surface.  Hence  we  have  summer  in  the 
northern  hemisphere  in  July,  when  the  earth  is  farthest 
from  the  sun  ;  and  winter  in  January,  when  she  is  near- 
est the  sun  (144). 

Though  nearer  the  sun  in  January  than  in  July,  still, 
as  the  northern  hemisphere  is  then  inclined  from  the 
sun,  his  rays  strike  its  surface  obliquely  ;  less  light  falls 
upon  the  same  space  than  if  its  contact  wTere  mwe  direct, 
and  it  is  consequently  cold.  But  in  July,  the  rays  are 
more  direct — the  northern  hemisphere  being  inclined 
toward  the  sun — and  it  is  summer,  notwithstanding  we 
are  three  millions  of  miles  further  from  the  sun  than  in 
January. 

1.  The  comparative  amount  of  light  received  in  the  northern  hemisphere  In  July  and 
January  may  be  illustrated   by  the  accompany- 
ing figure,  in  which  the   rays   of  light  at   dif-          BUMMER  AND  WINTER  BATS. 
ferent  seasons  are  represented   to  the  eye.      In 

January,  they  are  seen  to  strike  the  northern 
hemisphere  obliquely,  and  consequently  the  same 
amount  of  light  is  spread  over  a  much  greater  sur- 
face. In  July,  the  rays  fall  almost  perpendicularly 
upon  us,  and  are  much  more  intense.  Hence  the 
variations  of  temperature  which  constitute  the 
seasons. 

2.  If  the  student  is  not  perfectly  clear  as  to  how   . 
the  north  pole  is  turned  first  toward  and  then  from 
tlie  sun,  he  will  need  to  be  guarded  against  the 
vulgar  idea  that  the  earth's  axis  "  Wabbles,"  as  it  is 
called.     By  consulting  119  to  121,  and  the  cuts,  it 
will  be  seen  that  the  very  permanency  of  a  plan- 
et's axis,  combined  with  its  periodic  revolution,  gives  the  beautiful  and  ever  welcome 
changes  of  the  seasons.    How  simple,  and  yet  how  effectual,  this  Divine  mechanism ! 

201.  As  the  inclination  of  the  axis  of  a  planet  and  the 
length  of  its  periodic  time  determine  the  extent  and 
character  of  its  zones,  and  the  length  of  its  seasons,  it- 
follows  that  where  these  are  known,  we  have  a  reliable 
clew  to  the  seasons  of  a  planet,  even  though  we  have 
neither  visited  nor  heard  from  it ;  and  as  we  do  not  know 
the  inclination  of  the  axis  of  Mercury,  we  have  no 
knowledge  of  his  seasons. 

200.  What  governs  the  general  temperature  of  the  planets  ?    The  tem- 
perature of  particular  zones  ?    What  result  from  this  la<st  ?    Why  not^wartn- 
est  in  Ja-nuary,  &c.  ?    (Illustrate  by  diagram.)     How  are  the  poles  shifted  to 
and  from  the  sun  ?    Do  the  poles  "  wabble !" 

201.  How  ascertain  the  character  of  the  seasons  of  distant  planets?    5<?j*- 
bons  of  Mercury  ? 


98  ASTRONOMY. 


202.  The  seasons  of  Venus  are  very  remarkable.     So 
great  is  her  inclination  (122),  that  her  tropics  fall  within 
15°  of  her  poles,  and  her  polar  circles  (as  if  to  retaliate 
for  the  trespass  upon  their  territory),  go  up  to  within  15° 
of  her  equator.     Thus  the  torrid  and  frigid  zones  over- 
lap each  other,  and  the   temperate   zone  is  altogether 
annihilated. 

The  period  of  Yenus  being  but  225  days  (72),  the  sun 
declines  in  that  time  from  her  equinoctial  to  within  15° 
of  one  pole;  then  back  to  the  equinoctial,  and  to  within 
15°  of  the  other  pole,  and  again  back  to  the  equinoctial. 
The  effect  of  this  very  great  inclination  is  to  give  eight 
seasons  at  her  equator  every  225  days. 

In  her  short  period  of  225  days,  the  sun  seems  to  pass  from  her  northern  solstice 
through  her  equinox  to  her  southern  solstice,  and  back  to  the  point  from  which  he 
started.  When  he  is  over  one  of  her  tropics,  it  is  winter  not  only  at  the  other  tropic, 
but  also  at  her  equator;  and  as  the  sun  pusses  over  from  tropic  to  tropic,  and  back  again 
every  225  days,  making  spring  at  the  equator  as  he  approaches  it,  summer  as  he  passes 
over  it,  autumn  as  he  declines  from  it,  and  winter  when  he  reaches  the  tropic,  it  follows 
that  at  her  equator  Venus  has  eiykt  waxon*  in  one  of  her  years,  or  in  225  of  our  days. 
Her  seasons,  therefore,  at  her  equator,  consist  of  only  about  four  weeks  of  our  time,"or 
28£  days ;  and,  from  the  heat  of  summer  to  the  cold  of  winter,  can  be  only  about  56 
days.  At  her  tropics,  she  has  only  lour  seasons  of  56  days  each. 

203.  The  polar  inclination  of  Mars  being  28°  42'  (122), 
his  torrid  zone  must  be  57°  40'  from  his  poles — leaving 
only  32°  40'  for  the  width  of  his  temperate  zone.     But 
as  his  year  consists  of  687  days,  his  four  seasons  must 
consist  of  about  172   days  each,  or  nearly  twice   the 
length  of  the  seasons  of  our  globe. 

204.  So  slight  is  the  inclination  of  the  axis  of  Jupiter 
to  his  orbit,  that  he  has  but  a  narrow  torrid  zone,  and 
small  polar  circles.     As  his  orbit  departs  from  the  plane 
of  the  ecliptic  only  1°  18'  (108),  and  his  axis  is  inclined 
to  his  orbit  only  3°  5',  it  follows  that  his  axis  is  nearly 
Derpendicular  to  the  ecliptic.     The  sun  never  departs 
more  than  3°  5'  from  his  equator ;  and  still,  as  Lis  peri- 
odic time  is  about  12  years  (72),  he  has  alternately  six 
years  of  northern  and  six  of  southern  declination.     His 
narrow  torrid  zone  and  small  polar  circles  leave  very  ex- 

202.  Seasons  of  Venus  ?    Where  her  tropics  ?    Polar  circles  ?    Temperate 
cone  ?    Sun's  declination  upon  her  ?    Its  effect  ?    (Substance  of  note  () 

203.  Zones  of  Mars  ?    Length  of  seasons,  and  why  ? 

204.  Zones  of  Jupiter,  and  why  ?    Describe  his  climate.    Seasons  ?    Days 
uvi  uights  ?    Pole*  ? 


SEASONS   OF   THE   DIFFERENT   PLANETS     ETC. 


tensive  temperate  zones.  In  passing  from  his  equator  to 
his  poles,  we  meet  every  variety  of  climate,  from  the 
warmest  to  the  coldest,  with  but  slight  variation^in  any 
latitude,  from  age  to  age.  His  days  and  nights  are  al- 
ways nearly  of  the  same  length,  as  the  sun  is  always 
near  his  equinoctial.  His  poles  have  alternately  six 
years  day  and  six  years  night. 

205.  The  polar  inclination  and  zones  of  Saturn  differ 
but  little  from  those  of  Mars  ;  but  his  seasons  are  greatly 
modified  by  the  length  of  his  periodic  time.     This  being 
about  30  years,  his  four  seasons  must  each  be  about  7-J 
years  long  ;  and  his  polar  regions  must  have  alternately 
15  years  day  and  15  years  night.    The  rings  of  Saturn, 
which  lie  in  the  plane  of  his  equator,  and  revolve  every 
10^-  hours,  are  crossed  by  the  sun  when  he  crosses  the 
equinoctial  of  the  planet.     During  the  southern  declina- 
tion of  the  sun,  which  lasts  fifteen  years,  the  south  side 
of  the  rings  is  enlightened,  and  has  its  summer.     It  has 
also  its  day  and  night,  by  revolving  in  a  portion  of  the 
planet's  shadow.     When  the  sun  is  at  the  southern  tropic, 
it  is  midsummer  on  the  south  side  of  the  rings,  as  the 
rays  of  light  then  fall  most  directly  upon  them.     As  the 
sun  approaches  the  equator,  the  temperature  decreases, 
till  he  crosses  the  equinoctial,  and  the  long  winter  of  fif- 
teen years  begins.     At  the  same  time,  the  north  side  of 
the  rings  begins  to  have  its  spring ;  summer  ensues,  and 
in  turn  it  has  fifteen  years  of  light  and  heat.     The  influ- 
ence of  these  wonderful  rings  upon  the  climate  of  Saturn 
must  be  very  considerable.     During  the  winter  in  each 
hemisphere,  they  cast  a  deep  shadow  upon  some  portion 
of  his  surface  during  the  day  ;  and  in  the  summer,  these 
immense  reflectors  so  near  the   planet,   and  so  bright 
in  the  sunlight,  must  contribute  greatly  to  the  light,  if 
not  to  the  warmth,  of  his  summer  evenings.     The  poles 
of  Saturn  are  alternately  15  years  in  the  light,  and  15 
years  in  darkness. 

206.  Of  the  inclination  of  the  axes  of   Uranus  and 

205.  Zones  of  Saturn,  and  why?    Length  of  seasons?    Rings— how  *n- 
listened  ?    Influence  upon  climate  ?    Polar  days  and  nights  ? 


100  ASTRONOMY. 


Neptune,  respectively,  we  have  no  knowledge,  and  con 
sequently  can  form  no  opinion  respecting  their  tropics, 
polar  circles,  zones,  &c.  If  not  too  much  inclined,  like 
V enns,  they  have  but  four  seasons  in  their  year,  which 
would  make  each  season  of  Uranns  22  years  and  9  days 
long,  and  each  season  of  Neptune  41  years  and  56^  days 
long ;  as  these  periods  are,  respectively,  one-fourth  of  the 
periodic  time  of  the  planet  (72). 

Thus  we  see  that  tropics,  polar  circ'es,  zones,  ami  seasons  are  not  peculiar  to  our  globe, 
but  are  a  necessary  result  of  an  inclined  axis,  and  a  revolution  around  the  sun.  Tbo 
censes  which  produce  our  seasons  are  known  to  be  in  operation  in  other  planetary 
•worlds,  and  it  would  be  unreasonable  to  deny  that  tho  effect  was  there  also. 

DISCOVERY    OF    THE    DIFFERENT    PLANETS. 

207.  The  old  planets,  as  they  are  called,  viz.,  Mercury, 
Venus,  Mars,  Jupiter,  and  Saturn,  have  been  known  as 
planets,  or  "  wanderers,"  from  the  earliest  ages.     Uranus 
was  discovered  by  Sir  William  Herschel,  March  13th, 
1781.     Neptune  was  demonstrated  to  exist  before  it  had 
leen  seen,  by  M.  Le  Terrier,  of  France,  August,  1846; 
and  first  seen  by  Dr.  Galle,  of  Berlin,  Sept,  23,  1846. 

208.  The  discovery  of  Neptune  is  probably  one  of  the 
greatest   achievements    of  mathematical   science   ever 
recorded.     By  comparing   the   true   places   of  Uranus 
with  the  places  assigned  by  the  tables,  it  was  found  that 
he  was  not  where  his  known  rate  of  motion  required 
him  to  be ;  and  after  making  all  due  allowance  tor  the 
attraction  of  Jupiter  and  Saturn  (65),  by  which  pertur- 
bations would  be  produced,  it  was  found  that  there  was 
evidently  the  effect  of  some  other  body,  exterior  to  the 
orbit  of  Uranus,  the  attraction  of  which  body  helped  to 
cause  the  perturbations  of  Uranus.      From  this  effect^ 
produced  by  an  unknown  and  invisible  world,  lying  far 
3ut  beyond  the  supposed  boundaries  of  the  solar  system, 
not  only  was  the  existence  of  its  cause  demonstrated,  but 
its   direction,   distance,  mass,  and  period  were  proxi- 
mately  ascertained. 

206.  What  said  of  the  seasons  of  Urauus  and  Neptune  ?    Probable  length 
oi  former?     Latter?     (Remark  in  note  ?) 

207.  What  said  of  the  "  old  planets  ?"     Of  Uranus  ?    Neptune  ? 

208.  Describe  the  discovery  of  Neptune.     Perturbations  ?    Tables,  &c.  * 
(.Describe  successive  steps  in  detail.     "What  said  of  Mr.  Adams  *; 


DISCOVERY    OF   THE   DIFFERENT   PLANETS.  101 


1.  On  the  evening  of  the  23d  of  September,  1S4G.  Dr.  Gaile,  one  of  the  astronomer! 
of  the  Koyal  Observatory  at  Berlin,  received  a  letter  from  Le  Terrier,  of  Paris,  request- 
ing him  to  employ  the  great  telescope  at  his  command  in  searching  for  The  supposed 
new  planet,  and  giving  its  position,  as  ascertained  by  calculation,  as  325°  52'S'  of  geocen- 
tric longitude.     Dr.  Galle,  taking  advantage  of  the  very  evening  on  which  he  received 
Le  Terrier's  letter,  soon  discovered  an  object  resembling  a  star  of"  the  eighth  magnitude, 
near  the  spot  indicated  by  Le  Terrier,  as  the  place  of  the  new  planet     On  consulting  an 
accurate  star  chart,  it  was  found  that  no  such  star  was  there  laid  down,  and  observations 
were  at  once  commenced,  with  a  view  to  detecting  any  change  of  place.    In  three  hours 
time,  it  was  seen  to  have  moved ;  and  by  the  next  evening  at  eight  o'clock,  it  was  found 
to  have  retrograded  more  than  four  seconds  of  time  (see  97  and  cut) — a  circumstance 
which  proved  it  to  be  much  nearer  the  earth  than  the  fixed  stars,  and  consequently  a 
planet — the  very  planet  which  had  caused  the  unaccountable  irregularities  of  Uranus. 
The  geocentric  longitude  of  the  planet,  at  midnight,  September  23^"  1S4G,  was  325O52-S' ; 
wlaoh  was  less  than  1°  from  the  place  assigned  to  it  by  Le  Terrier!     The  reason  why 
Le  Terrier  wrote  to  Dr.  Galle  was,  that  the  former  had  no  suitable  telescope  for  con- 
ducting the  search  in  which  he  was  so  deeply  interested. 

2.  It  is  worthy  of  remark  that  Mr.  Adams,  of  St  John's  College,  Cambridge.  Eng- 
lanJ,  had  also  calculated  the  place,  &c.,  of  the  new  planet,  and  had  arrived  at  results 
similar  to  those  reached  by  Le  Terrier;  but  as  the  latter  had  published  his  conclusions 
first,  tho  honor  of  the  discovery  is  generally  accorded  to  Le  Terrier. 

209.  The  Asteroids  have  all  been  discovered  during 
the  present  century,  and  most  of  them  since  1847.  The 
first  known  was  discovered  by  Professor  Piazzi,  of 
Palermo,  on  the  first  day  of  January,  1801,  while 
searching  for  a  star  which"  he  found  mapped  down  in 
his  star- chart,  but  could  not  find  in  the  heavens.  He 
soon  lost  sight  of  it,  however,  on  account  of  its  nearness 
to  the  sun,  but  it  was  re-discovered  by  Dr.  Olbers,  of 
Bremen,  in  January,  1802.  Pallas,  Juno,  and  Yesta 
were  discovered  between  1802  and  1807,  after  which 
no  additional  asteroids  were  discovered  for  thirty-eight 
years,  or  till  1845.  From  1845  to  1855,  thirty  new 
asteroids  were  discovered,  while  fifty  others  were  added 
to  the  list  during  the  next  ten  years.  From  the  proba- 
bility that  they  are  very  numerous,  and  the  rapidity  with 
which  they  have  recently  been  discovered,  it  is  not 
unlikely  that  the  list  may  yet  be  extended  to  hundreds. 
(For  complete  list,  see  page  247.) 

209.  How  long  hare  asteroids  been  known  ?  When  and  by  whom  was  the 
first  discovered  ?  The  next  three  ?  How  many  from  1807  to  1845  ?  From 
1845  to  1855?  From  1855  to  1865?  From  1865  to  the  present  time!' 
What  said  of  the  probability  of  other  discoveries? 


102  ASTRONOMY 


CHAPTER   V. 

SECONDARY     PLANETS THE     MOON. 

210.  THE  Secondary  Planets  are  those  that  revolve 
statedly  around  the  primaries,  and  accompany  them  in 
their  periodical  journeys  around  the  sun.     Of  these,  the 
earth  has  one ;  Jupiter,  four ;    Saturn,  eight ;   Uranus, 
six;  and  Neptune,  one — in  all  twenty.     Besides  these, 
there  is  a  strong  suspicion  among  astronomers  that  Yenus 
is  attended  by  a  satellite,  and  that  Neptune  has  at  least 
two,  instead  of  one. 

Sir  John  Ilerschel  says  Uranus  is  attended  "  certainly  by  four,  and  perhaps  by  six, 
and  Neptune  by  two  or  more."  Outlines,  Art.  533.  In  regard  to  Venus,  Prof!  Hind,  of 
London,  says:  "Astronomers  are  by  no  means  satisfied  whether  Venus  should  be  at- 
tended by  a  satellite  or  not.  *  *  "*  It  is  a  question  of  great  interest,  and  must  re- 
main opea  for  future  discussion." 

211.  Though  the  secondary  planets  have  a  compound 
motion,  and  revolve  both  around  the  sun  and  around 
their  respective  primaries,  they  are  subject  to  the  same 
general  laws  of  gravitation — of  centripetal  and  centrif- 
ugal  force — by   which   their  primaries    are    governed. 
Like  them,  they  receive  their  light  and  heat  from  the 
sun,  and  revolve  periodically  in  their  orbits,  and  on  their 
respective  axes.     In  the  economy  of  nature,  they  seem 
to  serve  as  so  many  mirrors  to  reflect  the  sun's  light  upon 
superior  worlds,  when  their  sides  are  turned  away  from 
a  more  direct  illumination. 

The  design  of  all  the  secondaries  may  be  inferred  from  what  is  said  of  the  purposes 
for  which  our  own  satellite  was  created.  "  And  God  said,  Let  there  be  lights  in  the  fir- 
mament of  heaven,  to  divide  the  day  from  the  night;  and  let  them  be  for  signs,  and  for 
seasons,  and  for  days  and  years ;  and  let  them  be  for  lights  in  the  firmament  of  heaven, 
to  give  light  upon  the  earth:  and  it  was  so.  And  God  made  two  great  lights;  the 
greater  light  to  rule  the  day,  and  the  lesser  light  to  rule  the  night:  he  made  the  stars 
also."— Geu.  i ,  14—16. 

210.  What  are  the  Secondary  planets  ?    How  many  ?    How  distributed  ? 
What  supposition  respecting  Venus  ?   Neptune?   (Ilerschel's  remark  ?  Prof. 
Hind's  ?) 

211.  What  said  of  the  laws  by  which  the  primaries  are  governed  ? 
and  heat?    Uses?    (From  what  may  we  infer  their  design?) 


THE   MOON.  103 


212.  To  the  inhabitants  of  our  globe,  the  earth's  satel- 
lite or  moon  is  one  of  the  most  interesting  objects  in  all 
the  heavens.     Her  nearness  to  the  earth,  and  .^consequent 
apparent  magnitude,  her  rapid  angular  motion  eastward, 
her  perpetual  phases  or  changes,  and  the  mottled  appear- 
ance of  her  surface,  even  to  the  naked  eye,  all  conspire 
to  arrest  the  attention,  and  to  awaken  inquiry.     Add  to 
this  her  connection  with  Eclipses,  and  her  influence  in 
the  production  of  Tides  (of  both  of  which  we  shall  speak 
hereafter  in  distinct  chapters),  and  she  opens  before  us 
one  of  the  most  interesting  fields  of  astronomical  re- 
search. 

213.  The  Romans  called  the  moon   Luna,   and  the 
Greeks  Selene.     From  the  former,  we  have  our  English 
terms  lunar  and  lunacy.     In  mythology,  Selene  was  the 
daughter  of  Helios,  the  sun.     Our  English  word  selenog- 
raphy— a  description   of  the  moon's   surface — is  from 
Selene,  her  ancient  name,  and  grapho,  to  describe. 

214.  The  point   in   the   moon's  APOGEE. 
orbit  nearest  the  earth   is   called 

Perigee,  from  the  Greek  peri, 
about,  and  ge,  the  earth.  The  point 
most  distant  is  called  Apogee,  from 
apo,  from,  and  ge,  the  earth.  These 
two  points  are  also  called  the  ap- 
sides of  her  orbit ;  and  a  line  join- 
ing them,  the  line  of  the  apsides. 

See  the  moon  in  apogee  and  perigee  in  the  cut  The 
singular  of  apsides  is  apsis. 

215.  The  mean  distance  of  the 
moon  from  the  earth's  center  is,  in 

round  numbers,  239,000  miles ;  or,  more  accurately, 
238,650.  The  eccentricity  of  her  orbit  amounting  to 
13,333  miles,  of  course  her  distance  must  vary,  arid  also 
her  apparent  magnitude  (56).  Her  average  angular 

212.  What  said  of  our  moon  ?     Why  specially  interesting  ? 

213.  Latin  name  of  the  moon  1    Greek?    Derivation  of  words  from  Luna? 
Who  was  Selene  in  mythology  ?     Selenography  ? 

214.  Perigee  aoct  Apogee  1    Derivation?    What  other  name  for  these  two 
points  ?     What  is  the  line  of  the  apsides  ?     (Apsis  ?) 

215.  Moou's  distance?     Does  it  vary?    "Wh/?     Eccentricity  of  orbit? 


104  ASTRONOMY. 


diameter  is  31'  7",  and  Ler  real  diameter  2,160  miles. 
She  is  consequently  only  ^th  part  as  large  as  the  earth, 
and  nnnArnnr^  Part  as  large  as  the  sun. 

The  masses  of  globes  are  proportional  to  the  cubes  of  their  diameters.  Then 
2,160  X  2, ICO  X  2.160  =  10,077,696,000,  the  cube  of  the  moon's  diameter;  and  7,012 
X  7,912x7,912  =  495,239,174. 428,  the  cube  of  the  earth's  diameter.  Divide  the  latter 
by  the  former,  and  we  have  49  and  a  fraction  over,  as  the  number  of  times  the  bulk  of 
the  moon  is  contained  in  the  earth.  Its  mass,  as  compared  with  the  sun,  is  ascertained 
in  the  same  manner. 

216.  The  plane  of  the  moon's  orbit  is  very  near  that 
of  the  ecliptic.  It  departs  from  the  latter  only  about 
5  J°  (5°  8'  48"). 

INCLINATION  OF  TUB  MOON'S  ORBIT  TO  TH3  PLANE   OF  THE  ECLIPTIC. 


MOTION  OF  TUB  APSIDES. 


Let  the  line  A  B  represent  the  plane  of  the  earth's  orbit,  and  the  line  joining  the 
moon  at  C  and  D  would  represent  the  inclination  of  the  moon's  orbit  to  that  of  the 
earth.  At  C  the  moon  would  be  within  the  earth's  orbit,  and  at  D  exterior  to  it ;  and 
it  would  be  Full  Moon  at  D,  and  New  Moon  at  C. 

217.  The  line  of  the  apsides  of  the  moon's  orbit  is  not 
fixed  in   the  ecliptic,  but  revolves   slowly  around  the 
ecliptic,  from  west  to  east, 

in  the  period  of  about  nine 
years. 

In  the  adjoining  cut,  an  attempt  is  made 
to  represent  this  motion.  At  A,  the  line 
of  the  apsides  points  directly  to  the  right 
and  left;  but  at  B,  C,  and  D ,  it  is  seen 
changing  its  direction,  till  at  E  the  change 
»s  very  perceptible  when  compared  with 
A.  But  the  same  ratio  ot  change  con- 
tinues; and  at  the  end  of  a  year,  when  the 
earth"  readies  A  again,, the  line  of  the  ap- 
sides is  found  to  have  revolved  eastward 
to  the  dotted  line  I K,  or  about  40°.  In 
jine  years,  the  aphelion  point  near  A  will 
have  made  a  complete  revolution,  and  re- 
turned to  its  original  position. 

218.  The  line  of  the  moon's  nodes  is  also  in  revoln 
tion  ;  but  it  retrogrades  or  falls  back  westward,  making 
the  circuit  of  the  ecliptic  once  in  about  19  years. 

Angular  diameter?  In  miles  ?  How  compare  with  earth  ?  With  sun?   (Ilo^r 
demonstrated  ?) 

216.  How  is  the  plane  of  the  moon's  orbit  situated  with  respect  to  the 
ecliptic?    (Illustrate  by  diagram.) 

217.  Is  the  line  of  the  moon's  apsides  stationary  or  not  ?    What  motion  1 
Period  ?    (Illustrate.) 

218.  What  of  the  line  of  the  moon's  nodes  ?    la  what  time  d-?es  it  mako 
the  circuit  of  the  ecliptic  ?    Amount  of  motion  ? 


THE   MOON.  105 


The  amount  of  this  motion  is  10°  35'  per  annum,  which  would  -"quire  IS  years  aiu 
21U  days  for  a  complete  revolution. 

219.LThe  diameter  of  the  moon  is  only  7Joth 
part  as  great  as  that  of  the  sun  ;  and  yet  the  ap- 
parent diameter  of  the  moon  is  nearly  equal  to 
that  of  the  sunT7  The  former  is  31'  7",  and  the 
latter  32'  2",  orurily  55"  difference.  The  reason 
why  the  moon  appears  to  vie  with  the  sun  in 
magnitude,  when  she  is  only  T7j-0  o-V,¥^o  as  large, 
is,  that  she  is  400  times  nearer  to  us  than  he  is. 
See  Art.  56. 

1.  Tlie  cut  in  the  margin  will  show  how  it  is  that  a  small  object  near  us  will 
fill  as  large  an  angle,  or,  in  other  words,  appear  as  large,  as  a  much  larger 
object  which  is  more  remote.    The  moon  at  A  fills  the  same  angle  that  is 
filled  by  the  sun  at  B. 

2.  This  fact  may  serve  to  illustrate  the  comparative  inflnance  of  things 
present  and  future  upon  most  minds.    The  little  moon  may  eclipse  the  sun ; 
or  even  a  dime,  if  held  near  enough  to  the  eye,  will  completely  hide  all  his 
glories  from  our  view.    So  in  morals  and  religion.     The  "  things  which  are 
seen  and  temporal"  are  too  apt  to  hide  from  our  view  the  more  distant  but 
superior  glories  of  the  lite  to  come. 

220.  The  density  of  the  moon  is  only  about 
two-thirds  that  of  the  earth,  and  her  surface  ^th 
as  great.     The  light  reflected  to  the  earth  by  her, 
at  her  full,  is  only  3-00/00  otn  Pai't  as  much  as  we 
receive  oh  an  average  from  the  sun. 

221.  The   daily    apparent  revolution   of  the  . 
moon  is  from  east  to  west,  with  the  sun  and          f\ 
stars ;  but  her  real  motion  around  the  earth  is  V  "  •' 
from  west  to  east.     Hence,  when  first  seen  as  a  "  new 
moon,"  she  is  very  near,  but  just  east  of  the  sun ;  but 
departs  further  and  further  from  him  eastward,  till  at 
length  she  is  seen  in  the  east  as  a  full  moon,  as  the  sun 
goes  down  in  the  west. 

222.  The  moon  performs  a  sidereal  revolution  around 
the  earth  in  27d.  7h.  43m. ;  and  a  synodic  in  29d.  12h. 
44m.     The  sidereal  is  a  complete  revolution,  as  measured 
by  a  fixed  star ;  but  the  motion  of  the  earth  eastward  in 

219.  Moon's  diameter,  as  compared  with  that  of  the  sun?    With  sun's 
apparent  diameter  ?    Why  appear  so  near  of  a  size  ?    (Illustrate  by  diagram. 
Reflection  of  the  author?) 

220.  Density  of  the  moon  ?    Her  lurht  ? 

221.  Her  daily  apparent  motion  ?    Real  motion  ?    How  traced  ? 

222.  Wbat  is  her  sidereal  revolution?     Her  synodic?     What  difference? 
Why  ?    (illustrate  by  diagram.) 

5* 


106 


ASTKONOMY. 


her  orbit  gives  the  sun  an  apparent  motion  eastward 
among  the  stars  (119),  and  renders  it  necessary  for  the 
moon  to  perform  a  little  more  than  a  complete  revolution 
each  month,  in  order  to  come  in  conjunction  with  the 
sun,  and  make  a  synodic  revolution. 

SIDEREAL  AND  SYNODIC  REVOLUTIONS  OF  THE  MOON. 


_S[DERCAL_RCVOiUTION    2.7i;DAYS  -B/P        fig.  \      \ 

- XS3J?  ' 


SUN  AND   MOON    IN  CONJUNCT  10  N  -  NEW  MOON/ 


1.  On  the  right,  the  earth  is  shown  in  her  orbit,  revolving  around  the  sun,  and  the 
tnoon  in  her  orbit,  revolving  around  the  earth.    At  A,  the  sun  and  moon  are  in  con- 
junction, or  it  is  Ne^o  Moon.     As  the  earth  passes  from  D  to  E,  the  moon  passes 
around  from  A  to  B,  or  the  exact  point  in  her  orbit  where  she  was  27j  days  before. 
But  site  is  still  west  of  the  sun,  and  must  pass  on  from  B  to  C,  or  1  day  and  20  hour* 
longer,  before  she  can  again  come  in  conjunction  with  him.    This  1  day  and  20  hours 
constitutes  the  difference  between  a  sidereal  and  a  synodic  revolution. 

2.  The  student  will  perceive  that  the  difference  between  a  sidereal  and  synodic'revo- 
lution  of  the  moon,  like  that  between  solar  and  sidereal  time,  is  due  to  the  same  cause — 
namely,  the  revolution  of  the  earth  around  the  sun.    See  135. 

223.    The  daily  angular        DA1LT  ™<>GB=SS  OF  THE  MOON  EASTWARD. 

motion  of  the  moon  east- 
ward  is  13°  10' 35".  Her 
average  hourly  motion  is 
about  32,300  miles.  This 
motion  may  be  detected 
by  watching  her  for  a  lew/ 
hours  only  ;  and  by  markets  t 
ing  her  position,  with  ref- 
ence  to  the  stars,  from 
night  to  night,  her  daily 
journeys  will  appear  pro- 
minent and.  striking. 

The  estimate  of  13°  10'  35"  is  made 

for  a  sidereal  day  of  twenty-four  hours.  In  the  above  cut,  the  daily  progress  of  the 
moon  may  be  traced  from  her  conjunction  or  "  change"  at  A  on  the  right,  around  to  tho 
suine  point  again.  This  being  a  sidereal  revolution,  requires  only^27£  days. 


19   •* 


223.  Daily  angular  motion  eastward  ?    How  detected  ?    (For  what  day  is 
Ibis  estimate  made  ?) 


THE   MOON. 


107 


224.  In  her  journey  ings   eastward,   the  moon   often 
seems    to  run   over  and  obscure 

the  distant  planets  and  stars. 
This  phenomenon  is  called  an  oc- 
cidtation. 

The  adjoining  cut  represents  the  new  moon  as 
just  atxuit  to  obscure  a  distant  star,  by  passing  be- 
tween us  and  it.  In  1850,  she  occulted  Jupiter  for 
three  revolutions  in  succession — viz_,  Jan.  3uth,  Feb. 
27th,  and  March  26th.  Through  a  telescoi>e,  tlie 
uioon  is  seen  to  be  constantly  obscuring  stars  that 
are  invisible  to  the  naked  eye.  They  disappear  be- 
hind the  moon's  eastern  iimb,  and  in  a  short  time 
reappear  from  behind  her  western  ;  thus  distinctly 
exhibiting  her  eastward  motiwn. 

225,  Though  the  moon's  orbit  is  an  ellipse,  with  res- 
pect to  the  earth,  it  is,  in  reality,  an  irregular  curve, 
always  concave  toward  the  sun, 

and   crossing  the  earth's  orbit 
every  13°  nearly. 


1.  If  the  earth   st<»od  still  in  her  orbit,  the 
moon  would  describe  just  such  a  path  in  the 
eoHptia  as  she  describes  with   respect  to  the 
earth. 

2.  If  the  earth  moved  but  slowly  on  her  way, 
the  moon  would  actually  retrograde  on  the  eclip- 
tic at  the  time  of  her  chanae^  and  would  cross 
lier  own  path  at  every  revolution,  as  shown  in 
the  adjoining  figure.     But  as  the  earth  advances 
8«>me  46  millions  of  miles.  <«•  near  UK)  times  tlie 
diameter  of  the  moon's  orbit,  during  a  single  lu- 
nation, it  is  evident  that  the  moon's  orbit  never 
can  return  into  itselfj  or  retrograde,  as  here  rep- 
resented. 


THE  MOON  8  ORBIT  ALWAYS  CONCAVE  TOWARD  THE  SUN. 


3.  That  the  lunar  orbit  is  always  concave  toward  the  sun,  may  be  demonstrated  by 
•he  above  diagram.  Let  the  upper  curve  line  A  B  represent  an  arc  of  the  earth's 
orbit,  equal  to  that  passed  through  by  the  earth  during  half  a  lunation.  Now  the 
radius  and  arc  being  known,  it  is  found  that  the  chord  A  B  must  pass  more  than  400,000 
miles  within  the  earth.  But  a*  the  moon  departs  only  240,000  from  the  earth,  as  shown 
in  the  figure,  it  follows  that  she  must  describe  the  curve  denoted  by  the  middle  line, 
which  is  concave  toward  the  sun. 


224.  What  are  occit7totwn.s ?    How  produced ?    (Are  they  frequent?    Are 
planets  ever  occulted?    Describe  process.) 

225.  What  is  the  form  of  tlie  moon's  orbit  with  respect  to  the  earth  ?    The 
sun  ?    (How  if  the  eurth  were  stationary  ?     If  moving  slowly  ?     Demon- 
strate her  orbit  to  be  concave,  &c.     Draw  orbit  for  complete  lunation,  an*3 
describe  her  relative  motion.) 


108 


ASTRONOMY. 


4  This  subject  may  be  still  further  illustrated  by  the  following  cut,  representing 

THE  MOON'S  PATH  DURING  A  COMPLETE  LUNATION. 

C  -  B 


MOON'S   PATH. 


Here  the  plain  line  represent?  the  earth's  orbit,  and  the  dotted  one  that  of  the  mo»>!i. 
At  A  the  moon  crosses  the  earth's  track  240,000  miles  behind  her.  She  gains  on  the 
cartii,  till  in  seven  days  she  passes  her  at  1>  as  a  Full  Moon.  Continuing  to  gain  oa 
the  eartli,  she  crosses  her  orbit  at  0,  240,000  miles  ahead  of  her,  being  then  at  her  Third 
Quarter.  From  this  point  the  earth  gains  upon  the  moon,  till  seven  days  afterward  she 
overtakes  her  at  D  as  a  New  Moon.  From  D  to  E  the  earth  continues  to  gain,  till  at 
E  the  moon  crosses  240,000  Behind  the  e<irth,  as  she  had  done  four  weeks  before  at  A. 
Thus  the  moon  winds  her  way  along,  first  within  and  then  without  the  eartli;  always 
gaining  upon  us  when  outside  of  our  orbit,  and  falling  behind  us  when  within  it. 

5.  The  t>mal!  circles  in  the  cut  represent  the  moon's  orbit  with  respect  to  the  earth, 
which  is  as  regular  to  ux  as  if  the  earth  had  no  revolution  around  the  snn. 

226.  The  moon  never  retrogrades  on  the  ecliptic,  or 
returns  into  her  own  path  again ;    but  is  always   ad- 
vancing with  the  earth,  at  the  rate 

of  not  less  than  65,700  miles  per 

Mrhe  moon's  orbitual  Telocity,  with  respect  to 
theearth,  is  about  2,300  miles  per  hour.  When  out- 
Bide  the  earth,  as  at  B,  in  the  last  figure,  she  gains 
2,300  miles  per  hour,  which,  added  to  the  earth's  ve- 
locity, would  give  7o,300  miles  as  the  hourly  velocity 
of  the  moon.  When  within  the  earth's  orbit,  as  at 
I),  she  loses  2,300  miles  per  hour,  which,  subtracted 
from  63,000  miles  (the  earth's  hourly  velocity),  would 
leave  65,700  miles  as  the  slowest  motion  of  the  moon 
in  space,  even  when  she  is  falling  behind  the  earth. 

2.  Could  we  look  down  perpendicularly  upon  the 
ecliptic,  and  see  the  paths  of  the  earth  and  moon, 
we  should  see  the  latter  pursuing  her  serpentine 
course,  first  within  and  then  outside  our  globe,  somewhat  as  represented  by  the  dotted 
line  in  the  annexed  figure.  Iler  path,  however,  would  be  concave  toward  the  sun,  as 
shown  on  the  preceding  page,  and  not  convex,  as  we  were  obliged  to  represent  it  hero 
in  so  small  a  diagram. 

227.  That  the  moon  is  opake,  like  the  rest  of  the  plan- 
ets, and  shines  only  by  reflection,  is  obvious,  from  the 
fact  that  we  can  see  only  that  part  of  her  upon  which 
the  sun  shines ;  and  as  the  enlightened  portion  is  some- 
times toward  and  sometimes  from  us,  the  moon  is  con- 
stantly varying  in  her  apparent  form  and  brightness. 
These  variations  are  called  her  phases. 

226.  At  what  rate  docs  the  moon  advance  with  the  earth  1    Moon'a  or 
bitual  velocity,  with  respect  to  the  earth  \    Slowest  motion  \    (Illustrate  the 
moon's  course.) 

227.  What  proof  that  the  moon  is  opake  1    What  meant  by  her  phases  f 


THE   MOON. 


109 


228.  The  cause  of  the  moon's  phases  —  her  waxing  and 
waning  —  is  her  revolution  around  the  earth,  which  ena- 
bles us  to  see  more  of  her  enlightened  side  at  one  time 
than  at  another. 


CAUSE   OF  THE  MOON'S  PHASES 


4 


1.  This  cut  represents  the  moon  revolving  eastward  around  the  earth.     In  the  outside 
circle,  she  is  represented  as  she.  would  appear,  if  viewed  from  a  direction  at  right  angles 
with  the  plane  of  her  orbit.    The  side  toward  the  sun  is  enlightened  in  every  case,  and 
she  appears  like  a  half  moon  at  every  point. 

2.  The  interior  suit  represents  her  as  she  appears  when  viewed  from  the  earth. 
At  A  it  is  New  Moon  ;  and  if  seen  at  all  so  near  the  sun,  she  would  appear  like  a  dark 
globe.     At  B  she  would  appear  like  a  crescent,  concave  toward  the  east.     At  0,  more 
of  her  enlightened  side  is  visible ;  at  D.  still  more ;  a.nd  at  E,  the  enlightened  hemisphere 
is  fully  in  view.     We  then  call  her' a  Full  Moon.     From  E  around  to  A  again,  the  dark 
portion  becomes  more  and  more  visible,  as  the  luminous  part  goes  out  of  view,  till  she 
comes  to  her  change  at  A.    When  at  I)  and  F,  the  moon  is  said  to  be  gibbous. 

3.  If  the  student  will  turn  his  book  bottom  upward,  and  hold  it  south  of  him,  he  will 
pee  -ucJiy  the  crescent  of  the  old  moon  at  II  is  concave  on  the  west,  instead  of  the  east, 
like  the  new  moon,  and  why  she  is  seen  before  sunrise,  instead  of  just  after  sunset 

229.  The  cusps  of  the  moon  are  the  extremities  of  the 
crescent.     Her  syzygies  are  two  points  in  her  orbit  180° 
apart,  where  she  is  new  and  full  moon.     (See  positions 
1  and  3  in  the  last  cut.)     The  quadratures  are  four  points 
90°  apart  (like   1,  2,  3,  and  4  in  cut) ;  and  her  octants 
eight  points  45°  apart  (like  A,  B,  C,  &c.,  in  the  cut). 

230.  The  moon  is  said  to  change  when  she  comes  in 
conjunction  with  the  sun,  and  is  changed  from  Old  Moon 
to  New  Moon. 


228.  Cause  of  phases  ?     (Illustrate.) 

229.  What  are  the  cnxps  of  the  moon?    Her  Syzygies ?    Quadratures  ?    Oc- 
tants?   (Illustrate  on  blackboard.) 

230.  What  meant  by  the  ckan,</e  of  the  moon?    (How  noticed  or  traced  ?) 


110  ASTRONOMY. 


If  the  student  will  be  on  the  look-out,  he  can  easily  find  the  moon  tcest  of  the  sun 
in  the  diti/time;  and.  by  observing  her  carefully,  will  see  that  she  is  rapidly  approach- 
ing him.  In  a  short  time*  she  will  be  lost  in  his  beams,  and  soon  after  will  appear  east 
of  the  sun,  just  after  sundown,  as  a  New  Moon.  This  change,  as  it  is  called,  takes  place 
when  she  passes  the  sun  eastward. 

231.  A  New  Moon  is  the  moon  when  she  has  jnst 
passed  the  sun  in  her  eastward  journey,  and  when  only 
a  small  portion  of  her  enlightened  hemisphere  is  visible 
from  the  earth.     She  then  appears  like  a  slender  cres- 
cent, concave  on  the  east.     The  First  Quarter  is  when 
she  has  advanced  90°  eastward  from  the  sun.     She  is 
then  south  of  us  at  sundown,  and  we  see  one-half  of  her 
enlightened  side.     The  Full  of  the  moon  is  when  she  has 
advanced  180°  from  the  sun,  and  is  in  the  east  when  he 
goes  down  in  the  west.     Her  enlightened  side  is  then 
toward  us,  and  she  appears  circular,  or  full.     The  Third 
Quarter  is  when  the  moon  has  advanced  270°,  or  Jths 
of  her  synodic  journey.     She  has  been  waning  since  the 
full,  on  her  western  limb,  and  is  now  gibbous.     She  is  but 
90°  west  of  the  sun,  is  approaching  him,  and  waning 
more  and  more  every  day.     The  waxing  of  the  moon  is 
from  the  change  to  the  full ;  and  the  waning,  from  the 
full  to  the  change  again. 

We  earnestly  recommend  to  both  teacher  and  student  to  observe  the  present  place 
and  appearance  of  the  moon,  and  watch  her  through  one  lunation  at  least  A  little  time 
spent  in  this  way  will  do  more  to  fix  correct  ideas  in  the  mind  than  months  of  abstract 
study. 

232.  The  line  which  separates  the  dark  from  the  en- 
lightened portion  of  the  moon's  disk  is  called  the  Termi- 
nator. 

As  just  one-half  of  the  moon  is  always  enlightened  by  the  snn,  whether  it  appears 
BO  to  us  or  not,  it  follows  that  the  terminator  must  extend  quite  around  the  moon, 
dividing  the  enlightened  from  the  unenlightened  hemisphere.  This  circle  is  called 
the  Circle  of  Illumination.  At  new  and  full  moon  this  circle  is  ti'Meici-ie  to  us ;  but 
at  the  first  and  third  quarters,  it  is  edyewise.  The  portion  of  the  terminator  visible 
from  the  earth  traverses  the  moon's  disk  twice  during  every  lunation. 

233.  A  variety  of  dark  lines  and  spots  may  be  seen 
upon  the  surface  of  the  moon  with  the  naked  eye.    There 
is  a  dark  figure  on  her  western  limb,  resembling  that  of 
a  man,  with  his  head  to  the  north,  and  his  body  inclined 

231.  What  is  the  New  Moon?     Hmv  appear?    First  Quarter?    When? 
Appearance?    Full  Moon  and  appearance^     Third  Quarter?    Position  and 
appearance  ?    When  waxing  ?     Waning  ?    (What  recommended  by  author  'i) 

232.  What  is  the  Terminutor ?    (Substance  of  note?) 

233.  Describe  the  natural  appearance  of  the  full  moon.    (What  said  of  cut  ? 
Sketeh  on  blackboard.    Ojibway  legend  ?) 


THE  MOON NATURAL  APPEARANCE. 


Ill 


irregular 


Just  east  of  him,  and  opposite  his  shoiLders* 
object,  resembling   a   huge  bundle  or 


NATURAL  APPEARANCE  OF  THE 
W1A.  MOON. 


to  the  east, 
is  an 

pack. 

1.  Both  these  objects  are  represented  in  the  ad- 
joining cut,  which  was  drawn  from  nature   by  the 
author,  on  the  evening  of  December  18,  1S50.     It 
represents  the  moon  as  she  appears  when  about  two 
hours  high,  and  is  the  best  of  six  different  sketches 
taken  during  the  same  evening.   Let  the  student  com- 
pare it  \\~\ti\  the  next  Full  Moon,  and  see  if  our  draw- 
ing is  correct. 

2.  The  Ojibway  Indians  have  a  legend  by  which 
they  explain  this  singular  appearance  of  the  moon. 
Instead  of  a  "  man,"  they  say  this  figure  is  a  beautiful 
Ojibway  maiden,  who  was  translated  to  the  moon 
"many  snows  ago."  for  having  set  her  affections  upon 
that  object,  and  refusing  to  marry  any  of  the  "young 
braves"  of  the  Ojibway  nation.    How  the  "  beautiful 
maiden"  came  to  look  so  coarse  and  masculine,  and 
what  the  rest  of  the  figure  means,  the  tradition  does 
not  inform  us. 

234.  Theee  rude  figures  upon  the   moon's  disk   are 
probably  the  outlines  of  her  great  natural  divisions,  as 
mountains,  valleys,  and  continents. 

Almost  everybody  has  noticed  these  rude  figures  upon  the  face  of  the  moon,  and 
'many,  doubtless,  have  wondered  what  they  were;  but  how  few  have  supposed,  as  they 
were  gazing  upon  her  mottled  disk,  that  they  were  enjoying  a  distant  view  of  a  world, 
and  that  these  dim  outlines  wen*  natural  map  of  its  nearest  hemisphere!  Having 
freen  the  '-man  in  the  moon."  they  have  supposed  it  useless  to  pursue  the  subject  any 
further,  and  here  their  investigations  have  ended. 

235.  By  a  careful  observation  of  the  moon's  disk,  from 
month  to  month,  it  is  found  that  the  same  side  is  always 
toward  the  earth.     From  this  fact,  it  follows  that  she  re- 
volves on  her  axis  but  once  during  her  synodic  revolu- 
tion around  the  earth. 


1.  By  watching  the  moon  carefully  with  the 
naked  eye,  it  will  be  seen  that  the  same  spots 
occupy  iiearly  the  same   places  upon  her  disk, 
from  month  to  month;    which  shows  that  the 
same  side  is  always  toward  us. 

2.  Suppose  a    monument    erected   upon   the 
moon's  surface,  so  as  to  point  toward  the  earth 
at  New  Moon,  as  represented  at  A.    From  the 
earth   it  would  appear    in   the    moon's  center. 
Now  if  the  moon  so  revolved  upon  her  axis,  in 
the  direction  of  the  arrows,  as  to  keep  the  pillar 
pointing  directly  toward  the  earth,  as  shown  at 
A,  B,  C,  and  D,  and  the  intermediate  points,  she 
must  make  just  one  revolution  on  her  axis  during 
her  periodic  revolution.     At  A,  the  pillar  points 
from  the  tun,  and  at  C  toward  him:  show'ng 
that,  in  going  half  way  round  the  earth,  she  has 
perfoimed  half  a  revolution  upon  her  axis. 


MOON'S  RKVOLUTION. 


cr 

/ 

«>c 


234.  What  are  these  rude  figures  supposed  to  be?    (INot< 

235.  "What  interesting  fact  established  by  watching  the  n 
lows  from  it  ?    (illustrate  by  sketch,  of  cut  on  blackboard.) 


(Note.) 
moon  ? 


What  fol- 


112  ASTRONOMY. 


236.  As  the  same  side  of  the  moon  is  always  toward 
us,  it  follows  that  the  earth  is  invisible  from  one-half  of 
the  moon.     From  the  other  half,  our  globe  would  appear 
like  a  stationary  planet,  nearly  thirteen  times  as  large  as 
the  moon  appears  to  us,  and  exhibiting  all  her  varying 
phases. 

237.  Though  the  moon  always  presents  nearly  the  same 
hemisphere  toward  the  earth,  it  is  not  always  precisely 
the  same.     Owing  to  the  ellipticity  of  her  orbit,  and  the 
conseqi  ent  inequality  of  her  angular  velocity,  she  ap- 
pears to  roll  a  little  on  her  axis,  first  one  way  and  then 
the  other — thus  alternately  revealing  and  hiding  new 
territory,    as   it   were,   on   her 

eastern  and  x  western  limbs. 
This  rolling  motion  east  and 
west  is  called  her  libration  in 
longitude. 

The  accompanying  cut  will  illustrate  the  sub- 
ject of  the  moon's  librations  in  longitude. 

1.  From  A  around  to  C,  the  angular  motion  is 
slmcer  than  the  average,  and  the  diurnal  motion 
gains  upon  it,  so  that  the  pillar  points  wcxtof  the 
earth,  and  we  see  more  of  the  eastern  limb  of 
the  moon. 

2.  From  C  to  A,  again,  the  moon  advances 

faster  than  a  mean  rate,  and   gains  upon  the  £ V  jy 

diurnal  revolution;   so  that  the  pillar  points  ea-fft  ^L/-^.          / 
of  the  earth,  and  we  see  more  of  the  moon's  "•--..*-  j«rk 

western  limb.     Thus  she  seems  to  lihrate  or  roll,  laj 

first  one  way  and  then  the  other,  during  every 
periodic  revolution. 
At  B,  we  see  most  of  her  eastern  limb ;  and  at  D,  most  of  her  western. 

238.  The  axis  of  the  moon  is  inclined  to  the  plane  of 
her  orbit  only  about  one  and  a  half  degrees  (1°  30'  1OS"). 
But  this  slight  inclination  enables  us  to  see  first  one  pole 
and  then  the  other,  in  her  revolution  around  the  earth. 
These  slight  rolling  motions  are  called  her  librations  in 
latitude. 

As  the  inclination  of  the  earth's  axis  brings  first  one  pole  and  then  the  other  toward 
the  sun,  and  produces  the  seasons,  so  the  inclination  of  the  moon's  axis  brings  first  one 
pole  and  then  the  other  in  view  from  the  earth.  But  as  her  inclination  is  only  Ij0,  the 
libration  in  latitude  is  very  slight. 

236.  What  other  fact  follows  from  the  moon's  keeping  the  same  side  toward 
us  ?    How  would  our  globe  appear  from  the  moon  \ 

237.  What  are  the  moon's  Vibrations  f    In  longitude,  and  cause?    (Illus- 
trate on  blackboard.) 

238.  In  latitude?    Cause  ?    (Illustrate  bv  the  case  of  the  earth.) 


THE     MOON TELESCOPIC     VIEW. 


113 


TELESCOPIC  VIEW   OF  THE  MOOW. 


239.  The  moon's  year  consists  of  29  J  of  our  days  ;  but 
as  she  makes  but  one  revolution  upon  her  axis  in  that 
time,  she  can  -have  but  one  day  and  one  night  in  her 
whole  year.     And  so  slight  is  the  inclination  of  her  axis 
to  the  plane  of  her  orbit,  that  the  sun's  declination  from 
her  equator  is  only  about  1-|°.     She  must  therefore  have 
perpetual  winter  at  her  poles  ;  while  at  her  equator,  her 
long  days  are  very  warm,  and  her  long  nights  very  cold. 

240.  By  the  aid  of  the  telescope,  the  surface  of  the 
moon  is  found  Jto  be  exceedingly  rough  and  uneven,  cov- 
ered with  vast  plains,  deep  valleys,  and  lofty  mountains. 
Several  of  the  latter  are  from  three  to  four  and  a  half 
miles  high.    That  they  are  really  mountains  is  proved  by 
three  facts :  1st,  the  line  of  the  terminator  is  jagged  or 
uneven,  as  shown  in  the  cut ;  2d.,  shadows  are  seen  pro- 
jecting first  to  the  east  and  then  to  the  west,  showing  the 
existence  of  elevations  of  some  sort,  that  intercept  the 
light ;  and  3d,  from  new  to  full  moon,  bright  spots  break 
out  from  time  to  time, 

just  east  of  the  ter- 
minator, in  the  dark 
portion,  and  grow 
larger  and  larger,  till 
they  join  the  illumi- 
nated portion,  show- 
ing them  to  be  the 
tops  of  mountains, 
which  reflect  the  sun- 
light before  it  reaches 
the  intervening  val- 
leys. 

1.  Specimens  of  these  s?iad- 
ows  may  be  ^een  in  the  cut.  pro- 
jecting to  the  leit.  Bright  points 
of  light,  or,  in  other  words,  the 
illuminated  tops  of  mountains, 
may  also  be  seen  near  the  terminator,  in  the  dark  portion.  The  writer  has  often 
•watched  them,  and  seen  them  enlarge  more  and  more,  as  the  sun  arose  upon  the  side  of 
the  moon  toward  us,  and  enlightened  the  sides  of  her  mountains. 

239.  Length  of  moon's  year  ?    Number  of  natural  days  ?    Sun's  declina- 
tion upon  her  ?    Climate  at  equator  and  poles  ? 

240.  How  appear  through  telescopes?    What  proof  of  mountains?    (Be- 
n>arks  upon  cut  ?    Observations  of  the  author  ?     Describe  shadows*  and  thei  r 
chui  ge&     Illustrate,  by  reference  to  the  Andes  and  their  shadows.) 


114:  ASTRONOMY. 


2.  The  shadows  are  always  projected  in  a  direction  opposite  the  sun,  or  toward  the 
dark  side  of  the  moon  ;  and  as  her  eastern  limb  is  dark  from  tup  change  to  the  full,  and 
her  western  from  the  full  to  the  change,  of  course  the  direction  of  the  shadows  must  be 
reversed. 

3.  Suppose  a  person  stationed  t;t  a  distance  directly  over  the  Andes.     Before  the 
6ii  n  arose,  he  would  see  the  tallest  peaks  enlightened;  and  as  he  arose,  the  long  shadows 
of  the  mountains  would  extend  to  the  west.    At  noon,  however,  little  or  no  shadow 
would  be  visible;  but  at  sunset,  they  would  again  be  seen  stretching  away  to  the  eaxt. 
This  is  precisely  the  change  that  is  seen  to  take   place   with    the  lunar  shadows,  except 
that  the  time,  required  is  a  lunar  day,  equal  to  about  15  of  our  days,  instead  of  one  of 
our  days  of  12  hours. 

241.  Some  of  the  lunar  mountains  are  in  extensile 
ranges,  like  our  Alps  and  Andes;  while  others  are  cir- 
cular, like  the  craters  of  huge  volcanoes.     Great  num- 
bers of  the  latter  may  be  seen  with  telescopes  of  only 
moderate  power.     Through  such  an  instrument,  the  moon 
will  appear  of  a  yellowish  hue,  and  the  circular  moun- 
tains like  drops  of  thick  oil  on  the  surface  of  water.   Two 
extensive  ranges,  and  several  of  the  circular  elevations, 
are  shown  in  the  last  cut.     Dr.  Scoresby,  of  Bradford, 
England,  who  examined  the  moon  through  the  monster 
telescope  of  Lord  Rosse,  says  he  saw  a  vast  number  of 
extinct  volcanoes,  some  of  whose  craters  were  several 
miles  in  breadth.     Her  general  appearance  was  that  of  a 
vast  ruin  of  nature.     Dr.  Herschel  supposed  he  saw  the 
light  of  several  active  volcanoes  upon  her  surface. 

242.  In   regard   to   the   existence  of  an  atmosphere 
around  the  moon,  astronomers  are  divided.     From  obser- 
vations   during   ellipses   of    the   sun,    and    other   phe- 
nomena, it  is  thought  that  if  the  moon  has  any  atmos- 

Ehere  at  all,  it  must  be  very  limited  in  extent,  and  far 
iss  dense  than  that  of  the  earth.     Dr.  Scoresby  saw  no 
indications  of  the  existence   of  water,  or  of  an  atmos- 
phere. 

From  observations  during  several  occultations  of  stars,  the  writer  is  of  opinion  that 
a  refracting  medium  of  some  sort  exists  in  the  vicinity  of  the  moon.  The  atm<infihf,re 
is  doubtless  subject  to  the  general  law  of  gravitation.  Hence  it  is  most  dense  at  the 
earth's  surface,  and  grows  rare  as  we  ascend.  Inasmuch,  therefore,  as  the  general  den- 
6ity  of  the  atmosphere  of  any  planet  is  dependent  upon  the  attracting  force  of  that 
planet,  and  the  moon  has  only  about  ~d  pan  as  much  attracting  power  as  the  earth,  it 
follows  tha^  her  atmosphere,  if  she  has  one,  ought  to  be  much  less  dense  than  ours. 

243.  That  no  water  exists  upon  the  moon's  surface, 

241.  Form  of  lunar  mountains  ?    Number  of  craters  visible?    Appearance 
,f  surface,  as  seen  by  Dr.  Scoresby  *     Dr.  Ilerschel's  supposition? 

242.  Has  the  moon  an  atmosphere  ?    Dr.  Scoresby's  statement  i    (Remark 
of  author  ?    Why  moon's  atmosphere  must  be  comparatively  rare  ?) 

243.  Why  .thought  there  is  no  water  on  the  moou  ? 


THE   MOON. 


115 


has  been  inferred  from  the  fact,  that  it  would  be  con- 
verted into  steam  or  vapor  during  her  long  and  hot  days, 
and  also  from  the  fact  that  no  clouds  are  ever  seen  float- 
ing around  her. 

244.  Professors  Baer  and  Madler,  of  Berlin,  have  con- 
structed a  map  of  the  moon,  which  is  characterized  by 
Professor  Nichol,  of  Glasgow,  as  "  vastly  more  accurate 
than  any  map  of  the  earth  we  can  yet  produce,"  and  as 
"the  only  authentic  and  valuable  work  of  the  kind  in 
existence." 

The  following  is  a  list  of  the  principal  lunar  mountains,  with  their  bight,  according 
to  the  recent  measurements  of  Madler: 


Feet. 

Posidonius 19,830 

Tycho 20,190 

Calippus 20,390 

Casatus 22,810 

Newton 23,830 


Miles. 

376 
3-83 
3-86 


Feet. 

Clavius 19,030 

Huygens 18,670 

Bl  an  can  us 18,010 


4-8-2  jMovetus      18,440 

4-52  I 


Milts. 
3-60 
3-54 
8-41 
3-49 


245.  The  apparent  position  of  the  moon  in  the  heavens 
is  one  of  the  principal  means  by  which  mariners  ascer- 
tain their  longitude  at  sea.  So  regular  is  her  motion, 
that  her  "place"  as  viewed  from 

any  fixed  point  on  the  earth,  at        "^- — Jl     o 

any  specified  time,  and  with  ref- 
erence to  the  four  stars  that  lie  in 
or  near  her,  may  be  determined 
for  months  and  years  to  come ; 
and,  by  observing  how  far  she  ap- 
pears out  of  place,  either  east  or 
west,  at  the  time  specified,  we 
may  determine  how  far  we  are 
east  or  west  of  the  place  for 
which  her  longitude  is  given  in 
the  tables. 

Let  A  in  the  cut  represent  Greenwich  Observa- 
tory, near  London.  B  is  the  moon,  and  C  her  appa- 
rent plaw  among  the  distant  stars,  about  4<io  west  of 
the  star  I).  The  ship  K,  having  Greenwich  time,  as 
wttll  as  her  own  local  time,  sails  from  London  west- 
ward.;  but  on  observing  the  moon  when,  by  Greenwich  time,  she  ought  to  he  at  C,  she 
is  fou'id  to  be  at  F,  or  only  about  20°  west  of  the  star  D.  It  is  therefore  obvious  that 

244.  What  celebrated  chart  mentioned  ?  How  characterized  ?  (What  lift 
of  mountains  given  ?  General  lii^ht  0 

24f>.  What  use  made  of  the  moon  in  navigation?  Explain  the  process* 
What  called  ?  What  otl:er  method  for  determining  longitude  ? 


116  ASTRONOMY. 


the  Khip  is  west  of  Greenwich,  as  the  moon  appears  east  of  her  Greenwich  place.  From 
this  difference  between  her  place  as  laid  down  in  the  tables,  and  her  observed  place,  a3 
referred  to  certain  prominent  stars,  the  manner  determines  how  far  he  is  east  or  west  of 
tlie  meridian  of  Greenwich.  The  moon's  geocentric  place  (or  place,  as  viewed  from  the 
center  of  the  earth)  may  be  given  instead  of  her  Greenwich  place,  and  the  same  conclu- 
sions arrived  at  In  either  case,  this  is  called  the  lunar  method  of  determining  the 
longitude.  It  is  also  ascertained  by  simple  comparison  of  local  and  standard  time,  as 
explained  at  151. 

246.  The  best  time  for  observing  the  inoon  with  a  tele- 
scope is  from  the  change  to  the  first  quarter,  and  from  the 
third  quarter  to  the  change.  Near  the  first  and  third 
quarters,  the  shadows  of  objects  are  seen  at  right  angles 
with  the  line  of  vision,  and  to  the  best  advantage  ;  wThile 
at  full  moon,  objects  cast  no  shadows  visible  to  us. 


CHAPTER   VI. 

ECLIPSES     OF    THE     SUN     AND     MOON. 

247.  An  Eclipse  is  a  partial  or  total  obscuration  or 
darkening  of  the  sun  or  moon,  by  the  intervention  of 
some  opake  body.  Eclipses  are  either  solar  or  lunar.  A 
solar  eclipse  is  an  eclipse  of  the  sun,  and  a  lunar  eclipse 
is  an  eclipse  of  the  moon.  A  solar  eclipse  is  caused  by 
the  moon,  when  she  passes  between  the  earth  and  the 
sun,  in  her  revolution  eastward,  and  casts  her  shadow 
upon  the  earth.  A  lunar  eclipse  takes  place  when  the 
moon  is  in  opposition  to  the  sun,  and  passes  through  a 
portion  of  the  earth's  shadow. 

The  general  law  of  shadows  may  be  illustrated  by  the  following : 


Here  the  sun  and  planet  are  represented  as  of  the  same  size  and  the  shadow  of  the 
latter  is  in  the  form  of  a  cylinder. 

246.  "When  is  the  best  time  for  viewing  the  moon  with  a  telescope  ?    Why  ? 

247.  What  is  an  eclipse?    A  solar  ?    Lunar?    Cause  of  solar  eclipses  ?    Of 
lunai  ?    When  do  lunar  eclipses  take  place  ?    (Illustrate  the  laws  of  shadows 
AY  Oiugram  on  blackboard.) 


ECLirSES   OF   THE   SUN   AND   MOON. 


117 


In  this  cut,  the  opake  body  is  the  larger,  and  the  shadow  projected  from  it  diverges 
r  grows  more  broad  as  the  distance  from  the  planet  increases. 


Here  the  luminous  "body  is  tJie  larger,  and  the  shadow  converges  to  a  point,  and  takei 
the  form  of  a  cone. 


Here,  also,  the  luminous  body  is  the  larger,  and  both  precisely  of  the  same  size  as  in 
the  cut  preceding;  but  being  placed  nearer  each  other,  the  shadow  is  shown  to  be  con- 
siderably shorter. 

24:8.  All  the  planets,  both  primaries  and  secondaries, 
cast  shadows  in  a  direction  opposite  the  sun  (see  the 
adjoining  cut)  The 
form  and  length  of  these 
shadows  depend  upon 
the  comparative  magni- 
tude of  the  sun  and 
planet,  and  their  dis- 
tance from  each  other. 
If  the  sun  and  a  planet 
were  of  the  same  size, 
the  shadow  of  the 
planet  would  be  in  the 
form  of  a  cylinder, 
whatever  its  distance. 
If  the  planet  were  Larger 
than  the  sun,  the  shad- 
ow would  diverge,  as 
we  proceed  from  the  planet  off  into  space;  and  the 
nearer  the  sun,  the  more  divergent  the  shadow  would  be. 


248.  What  said  of  the  shadows  of  the  planets  ?    Of  their/own  and  length  f 
[low  would  it  be  if  the  sun  and  planet  were  of  the  same  size  ?    If  the  planet 


118  ASTRONOMY. 


But  as  all  the  planets  are  much  smaller  than  the  sun, 
their  shadows  all  converge  to  a  point,  and  take  the  form 
of  a  cone  •  and  the  nearer  to  the  sun,  the  shorter  the 
shadow. 

These  principles  are  partly  illustrated  in  the  preceding  cut.  The  planets  nearest  the 
sun  have  comparatively  short  shadows,  while  those  more  remote  extend  to  a  great  dis- 
turice.  No  primary,  however,  casts  a  shadow  long  enough  to  reach  the  next  exterior 
planet 

249.  Eclipses  of  the  sun  must  always  happen  at  New 
Moon,  and  those  of  the  moon  at  Full  Moon.     The  reason 
of  this  is,  that  the  moon  can  never  be  between  us  arid 
the  sun,  to  eclipse  him,  except  at  the  time  of  her  change, 
or  new  moon ;  and  she  can  never  get  into  the  earth's 
shadow,  to  be  eclipsed  herself,  except  when  she  is  in  op- 
position to  the  sun,  and  it  is  full  moon. 

250.  If  the  moon's  orbit  lay  exactly  in  the  plane  of 
the  ecliptic,  she  would  eclipse  the  sun  at  every  change, 
and  be  eclipsed  herself  at  every  full  /  but  as  her  orbit 
departs  from  the  ecliptic  over  5°  (216),  she  may  pass 
either  above  or  below  the  sun  at  the  time  of  her  change, 
or  above  or  below  the  earth's  shadow  at  the  time  of  her 
full. 

NEW  AND  FULL  MOONS  WITHOUT  ECLIPSES. 
Shadow  abort  the  Earth.  Above  the  Earth's  shadow. 


Shadow  below  the  Earth.  Below  the  Earth's  shadow 

t.  Let  the  line  joining  the  earth  and  the  sun  represent  the  plane  of  the  ecliptic.  Now 
as  the  orl)it  of  the  moon  departs  from  this  plane  about  5°  0',  she  may  appear  either 
above  or  below  the  sun  at  new  moon,  as  represented  in  the  figure,  and  her  shadow  may 
fall  above  the  north  pole  or  below  the  south.  At  such  times,  then,  there  can  be  no 
solar  eclipse.  • 

2.  On  the  right,  the  moon  is  shown  at  her  full,  both  above  and  below  the  eartx'» 
shadow,  in  which  case  there  can  be  no  lunar  eclipse. 


was  largest?  If  brought  nearer  ?  How  if  planets  smallest  ?  How  affected 
by  distance  ?  (How,  then,  with  planets  nearest  the  sun  ?  More  remote  ' 
Does  any  primary  throw  its  shadow  out  to  the  next  exterior  planet?) 

249.  At  what  time  of  the  moon  do  solar  eclipses  always  occur?  Lunai * 
Why? 

250    Why  not  two  eclipses  every  lunar  month?    (IlluBtrate.) 


ECLIPSES   OF   THE   SUN   AND   MOON. 


119 


251.  Eclipses  of  the  sun  always  come  on  from  the  west, 
and  pass  over  eastward  ;  while  eclipses  of  the  moon  come 

™  f™™  the  Gas  t,  and  pass  over     BOLAB  ECLIP8E> 
•westward,    ibis  is  a  necessary 
result  of  the  eastward  motion 
of  the  moon  in  her  orbit. 

1.  In  the  right  hand  cut,  the  moon  is  seen  re- 
volving eastward,  throwing  her  shadow  upon 
the  earth,  and  hiding  the  western  limb  of  the 
sun.     In  some  Instances,  however,  when   the 
eclipse  is  very  slight,  it  may  first  appear  on  the 
northern  otfto-uifmm  limb  of  the  sun — that  is, 
the  upper  or  lower  side;   but  even   then  its 
direction  must  be  from  west  to  east.     It  will 
also  be  obvious  from  this  figure,  that  the  shad- 
ore  of  the  moon  upon  the  earth  must  also  trav- 
erse  her  surface   from   west  to  east;    conse- 
quently the  eclipse  will  be  visible  earlier  in  the 
west  than  in  the  east. 

2.  On  the  left,  the  moon  is  seen  striking  into 
the  earth's  shadow  from  the  west,  and  having 
her  eastern  limb  first  obscured.    By  holding 
the  book  up  south  of  him,  the  student  will  see 
at  once  why  the  revolution  of  the  moon  east- 
ward must  cause  a  solar  eclipse  to  proceed  from 
west  to  east,  and  a  lunar  eclipse  from  east  to 
west.    To  locate  objects  and  motions  correctly, 
the  student  should  generally  imagine  himself 
looking  to  the  south,  as  we  are  situated  north 
of  the  equinoctial.    The  student  should  bear  in 
mind  that  nearly  all  the  cuts  in  the  book  are 
drawn  to  represent  a  view  from  northern  lati- 
tude upon  the  earth.     Hence  by  holding  the 
book  up  south,  of  him,  the  cuts  will' generally 
afford  an  accurate  illustration  both  of  the  posi- 
tions and  motions  of  the  bodies  represented. 

252.  Eclipses  can  never  take  place,  except  when  the 
moon  is  near  the  ecliptic  ;  or,  in  other  words,  at  or  near 
one  of  her  nodes.     At  all  other  times,  she  passes  above 
or  below  the  sun,  and  also  above  or  below  the  earth's 
shadow.     It  is  not  necessary  that  she  should  be  exactly 
at  hei   node.,,  irk  jorder  that  an  eclipse  occur.     If  she  is 
withinmtf?  of  lifer  node  at  the  time  o£Jier  change,  she 
will  eclipse  the  sun ;  and  if  within  120<of  her  node  at  her 
full,  she  will  strike  into  the  earth's  shadow,  and  be  more 
or  less  eclipsed.     These  distances  are  called,  respectively, 
the  solar  and  lunar  ecliptic  limits. 

251.  What  is  the  direction  of  a  solar  eclipse  ?    A  lunar?    Why  this  dif 
fercrce? 

252.  Where  must  the  moon  be,  with  respect  to  the  ecliptic  and  her  nodes, 
in  order  to  an  eclipse  ?     What  meant  by  ecliptic  limits  ?    Name  the  distance 
uf  each,  respectively,  from  the  node.     (Illustrate.) 


120 


ASTRONOMY . 


This  subject  may  be  understood  by  consulting  the  following  figure: 

THE   MOON   CHANGING    AT   DIFFEKENT   DISTANCES    FROM    HER   NODI 


1.  Let  the  line  EE  represent  the  ecliptic,  and  the  line  OO  the  plane  of  the  moon's 
orbit.    The  light  globes  are  the  sun,  and  the  dark  ones  the  moon,  which  may  be  imag- 
ined as  much  nearer  the  student;  hence  their  apparent  diameter  is  the  same. 

2.  Let  the  point  A  represent  the  node  of  the  moon's  orbit.     Now  if  the  change  occur 
when  the  moon  is  at  B,  she  will  pass  belmo  the  sun.     If  when  at  C,  she  will  just  touch 
liis  lower  lirnb.     At  C,  she  will  eclipse  him  a  little,  and  so  on  to  A  ;  at  which  point,  if 
the  change  occurs,  the  eclipse  would  be  central,  and  probably  total. 

8.  If  the  moon  was  at  G,  H,  I,  or  J,  in  her  orbit,  when  the  change  occurred,  she  would 
eclipse  the  upper  or  northern  limb  of  the  sun,  according  to  her  distance  from  her  nodo 
at  the  time ;  but  if  she  was  at  K,  she  would  pass  above  the  sun,  and  would  not  eclipse 
him  at  all  The  points  C  and  J  will  represent  the  Solar  Ecliptic  Limits. 

253.  All  parts  of  a  planet's  shadow  are  not  alike  dense. 
The  darkest  portion  is  called  the  umbra^  and  the  partial 
shadow  the  penumbra. 

TJMBEA  AND   PENTTMBBA  OF  THE   EABTH  AND  MOON. 


Penumbra  is  from  the  Latin  pene,  almost,  and  vmlra,  a  shadow.  In  this  cut,  the 
earth's  umbra  and  penumbra  will  be  readily  found  by  the  lettering:  while  A  is  the  um- 
bra, and  B  B  the  penumbra,  of  the  moon.  The  latter  is  more  broad  than  it  should  be, 
owing  to  the  nearness  of  the  sun  in  the  cut,  as  it  never  extends  to  much  over  half  the 
earth's  diameter.  The  student  will  see  at  once  that  solar  eclipses  can  be  total  only  to 
persons  within  the  umbra;  while  to  all  on  which  the  penumbra  falls,  a  portion  of  the 
Bun's  disk  will  be  obscured. 

254.  The  average  length  of  the  earth's  umbra  is  about 
860,000  miles ;  and  its  breadth,  at  the  distance  of  the 
moon,  is  about  6,000  miles,  or  three  times  the  moon's 
diameter. 

As  both  the  earth  and  moon  revolve  in  elliptical  orbits,  both  the  above  estimates  are 
subject  to  variations.  The  length  of  the  earth's  umbra  varies  from  842.217  to  8?i,26« 
miles ;  and  its  diameter  There  the  moon  passes  it,  varies  from  5,235  to  6,365  miles. 

255.  The  average  length  of  the  moon's  umbra  is  about 
239,000  miles.     It  varies  from  221,148  to  252,638  miles, 


U53.  What  is  the  umbra  of  the  earth  or  moon  ?  The  penumbra  ?  (Deriva 
tioti  ?  Within  which  are  solar  eclipses  total  {\ 

254.  The  average  length  of  the  earth's  shadow  ?  Breadth  at  the  moonV 
distance  ?  (Do  they  vary  ?  Wrhy  I) 


ECLIPSES   OF  THE   SUN   AND   MOOX.  121 


according  to  the  moon's  distance  from  the  nm.  Its 
greatest  diameter,  at  the  distance  of  the  earth,  is  170 
miles ;  but  the  penumbra  may  cover  a  space  on  the 
earth's  surface  4,393  miles  in  diameter. 

256.  When  the  moon  but  just  touches  the  limb  of  the 
sun,  or  the  umbra  of  the  earth,  it  is  called  an  appulse. 
(See  D  and  G-,  in  the  first  cut  on  the  opposite  page.) 

A  partial  eclipse  is  one  in  which  only  part  of  the  sun 
or  moon  is  obscured.  A  solar  eclipse  is  partial  to  all 
places  outside  the  umbra ;  but  within  the  umbra,  where 
the  whole  disk  is  obscured,  the  eclipse  is  said  to  be 
total.  A  central  eclipse  is  one  taking  place  when  the 
moon  is  exactly  at  one  of  her  nodes.  If  lunar,  it  js 
total,  as  the  earth's  umbra  is  always  broad  enough,  at 
the  moon's  distance,  if  centrally  passed,  to  obscure  her 
whole  disk.  But  a  solar  eclipse  may  be  central  and  not 
total)  as  the  moon  is  not  always  of  sufficient  apparent 
diameter  to  cover  the  whole  disk  of  the  sun.  In  that 
case,  the  eclipse  would  be  annular  (from  annulus,  a 
ring),  because  the  moon  only  hides  the  center  of  the  sun, 
and  leaves  a  bright  ring  unobscured. 

PROGRESS  OF  A  CEKTEAL  -ECLIPSE. 
Going  off  Annular.  Coming  on. 


257.  It  has  already  been  shown  (50)  that  the  apparent 
magnitudes  of  bodies  vary  as  their  distances  vary  ;  and 
as  both  the  earth  and  moon  revolve  in  elliptical  orbits,  it 

255.  Average  length  of  the  moon's  umbra?    Does  it  vary  ?   Why  ?    Great- 
est diameter  at  the  earth's  surface  ?     Of  penumbra  ? 

256.  What  is  an  appnlw  f    A  partial  eclipse  ?    A  total  ?    A  central?    Arc 
all  central  eclipses  total  ?     Why  not  ?     What  called  then  ?     Why  ? 

257.  AVhy  are  some  central  eclipses  total,  and  others  par   al  and  annular? 
(Diagram.) 

6 


122  ASTRONOMY". 


follows  that  the  moon  and  sun  must  both  vary  in  their 
respective  apparent  magnitudes.  Hence  some  central 
eclipses  of  the  sun  are  total,  while  others  are  partial  and 
annular. 

TOTAL  AND  ANNTTLAB    ECLIPSES  OF  THB  SFW. 


1.  At  A,  the  earth  Is  at  her  aphtli&n,  and  the  sun  being  at  his  most  distant  point,  will 
have  his  least  apparent  magnitude.     At  the  same  time,  the  moon  is  in  perigee,  and  ap- 
pears larger  than  usual     If,  therefore,  she  pass  centrally  over  the  sun's  disk,  the  eclipse 
will  be  total. 

2.  At  B,  this  order  is  reversed.     The  earth  is  at  her  perihelion,  and  the  moon  in 
apogee;  so  that  the  sun  appears  larger,  and  the  moon  smaller  than  usual.     If.  then,  a 
central  eclipse  occur  under  these  circumstances,  the  moon  will  not  be  lanre  enough  tc 
eclipse  the  whole  of  the  sun,  but  will  leave  a  ring,  apparently  around  hevself,  unoh- 
scured.    Such  eclipse  will  be  annular. 

258.  As  the  solar  ecliptic's  limits  are  further  from  the 
moon's  nodes  than  the  lunar,  it  results  that  we  have  more 
eclipses  of  the  sun  than  of  the  moon.     There  may  be 
seven  in  all  in  one  year,  viz.,  five  solar  and  two  lunar; 
but  the  most  usual  number  is  four.     There  can  never  be 
less  than  two  in  a  year ;  in  which  case,  both  must  be  of 
the  sun.     Eclipses  both  of  the  sun  and  moon  recur  in 
nearly  the  same  order,  and  at  the  same  intervals,  at  the 
expiration  of  a  cycle  of  223  lunations,  or  18  years  of  365 
days  and  15  hours.     This  cycle  is  called  the  Period  of 
ike  Eclipses.     At  the  expiration  of  this  time,  the  suri 
and  the  moon's  nodes  will  sustain  the  same  relation  to 
each  other  as  at  the  beginning,  and   a   new  cycle   oi 
eclipses  begins. 

259.  In  a  total  eclipse  of  the  sun,  the  heavens  are 
shrouded  in  darkness,  the  planets  and  stars  become  visi- 
ble, the  temperature  declines,  the  animal  tribes  become 
agitated,  and  a  general  gloom  overspreads  the  landscape. 
Such  were  the  effects  of  the  great  eclipse  of  1806.     In  a 
lunar  eclipse,  the  moon  begins  to  lose  a  portion  of  her 

258.  Which  kind  of  eclipses  is  most  frequent?    Why?     The  greatest 
number  in  a  year  ?    How  many  of  each  ?     Least  number,  and  which  ?    Usu.\I 
number?    What  said  of  the  order  of  eclipses  ?    Time  of  cycle ? 

259.  Describe  the  effects  of  a  total  eclipse  of  the  smi.     The  process  of  » 
vnar  eclipse  ? 


ECLIPSES   OF   THE   SUN   AND   MOON.  123 

light  and  grows  dim,  as  she  enters  the  earth's  penumbra, 
till  at  length  she  comes  in  contact  with  the  umbra,  and 
the  real  eclipse  begins. 

260.  In  order  to  measure  and  record  the  extent  of 
eclipses,  the  apparent  diameters  of  the  sun  and  moon 
are  divided  into  twelve  equal  parts,  called  digits  /  and 
in  predicting  eclipses,  astronomers  usually  state  which 
"limb"  of  the  body  is  to  be  eclipsed — the  southern  01 
northern — the  time  of  the  first  contact,  of  the  nearest 
approach  of  centers,  direction,  and  number  of  digits 
eclipsed. 

FIVE  DIGITS   ECLIPSED.  TWELVE  DIGITS. 


261.  The  last  annular  eclipse  visible  in  the  United 
States  occurred    Oct.  19,  1865.     The  next  total  eclipse 
of  the  sun  will  be  August  7,  1869-. 

Some  of  the  ancients  and  all  barbarous  nations  formerly 
regarded  eclipses  with  amazement  and  fear,  as  supernatu- 
ral events,  indicating  the  displeasure  of  the  gods.  Colum- 
bus is  said  to  have  made  a  very  happy  use  of  this  supersti- 
tion. When  the  inhabitants  of  St.  Domingo  refused  to 
allow  him  to  anchor,  in  1502,  or  to  furnish  him  supplies,  he 
told  them  the  Great  Spirit  was  offended  at  their  conduct, 
and  was  about  to  punish  them.  In  proof,  he  said  the 
moon  would  be  darkened  that  very  night  /  for  he  knew 
an  eclipse  was  to  occur.  The  artifice  led  to  a  speedy  and 
ample  supply  of  his  wants. 

262.  Eclipses  can  be  calculated  with  the  greatest  pre- 
cision, not  only  for  a  few  years  to  come,  but  for  centuries 

260.  How  are  eclipses  measured  and  recorded  ? 

2tU.  When  the  next  annular  eclipse  visible  in  *liis  country?  The  next 
total  ?  How  have  the  ignorant  and  superstitious  regarded  eclipses  ?  Aneo- 
dote  of  Columbus  2 


121 


ASTRONOMY 


and  ages  either  past  or  to  come.  This  met  demonstrates 
the  truth  of  the  Copernican  theory,  and  illustrates  the 
order  and  stability  that  everywhere  reign  throughout  the 
planetary  regions. 


CHAPTER    VII. 


TELESCOPIC   VIEWS    OF  THE  MOONS  OP 
JUPITEK. 


SATELLITES     OF     THE     EXTERIOR     PLANETS. 

263.  JUPITER  is  attended  by  four  satellites  or  moons. 
They  are  easily  seen  with*  a  common  spy-glass,  appear- 
ing like  small  stars  near  the 

primary.  (See  adjoining  cut, 
and  note  at  178.)  By  watch- 
ing them  for  a  few  evenings, 
they  will  be  seen  to  change 
their  places,  and  to  occupy  dif- 
ferent positions.  At  times, 
only  one  or  two  may  be  seen, 
as  the  others  are  either  betwyeen 
the  observer  and  the  planet,  or 
leyond  the  primary,  or  eclipsed 
by  his  shadow. 

264.  The  size  of  these  satel- 
lites is  about  the  same  as  our 
moon,  except  the  second,  which 
is   a  trifle  less.     The   first   is 

about  the  distance  of  our  moon  ;  and  the  others,  respect- 
ively, about  two,  three,  and  five  times  as  far  off. 


COMPARATIVE  DISTANCES  OF  JUPITEE'S  MOONS. 


4th. 

•- 


3d. 


1st 


262.  What  said  of  the  calculation  of  eclipses  ? 
srratc  and  illustrate? 


What  does  this  demon- 


263.  Ilow  many  moons  has  Jupiter?   How  seen  ?   Why  not  all  seen  at  once  I 

264.  Their  size?    Distances?    Per 'da?     Why  so  rupi< 


ipid  ? 


SATELLITES    OF    THE    EXTERIOR    PLANETS.  125 


Their  periods  of  revolution  are  from  1  day  18  hours  to 
17  days,  according  to  their  distances.  This  rapid  mo- 
tion is  necessary,  in  order  to  counterbalance  the  power- 
ful centripetal  force  of  the  planet,  and  to  keep  the  satel- 
lites from  falling  to  his  surface. 

The  magnitudes,  distances,  and  periods  of  the  moons  of  Jupiter  are  as  follows : 

Diameter  in  mites.  Distance.  Periodic  times. 

lt,t 2,500  259,000  1  day  18  hours. 

2d 2,068  ...    414,000 3     "    12      "  * 

3d 3,377 647,000  7    "    14      " 

4th 2,890  1,164,000 1?    "     0      " 

265.  The  orbits  of  Jupiter's  moons  are  all  in  or  near 
the  plane  of  his  equator ;  and  as  his  orbit  nearly  coin- 
cides with  the  ecliptic,  and  his  equator  with  his  orbit,  it 
follows  that,  like  our  own  moon,  his  satellites  revolve 
near  the  plane  of  the  ecliptic.     On  this  account,  they 
are  sometimes  between  us  and  the  planet,  and  sometimes 
beyond  him,  and  seem  to  oscillate,  like  a  pendulum,  from 
their  greatest  elongation  on  one  side  to  their  greatest 
elongation  on  the  other. 

266.  Their  direction  is  from  west  to  east,  or  in  the 
direction  their  primary  revolves,  both  upon  his  axis  and 
in  his  orbit.     From  the  fact  that  their  elongations  east 
and  west  of  Jupiter  are  nearly  the  same  at  every  revolu- 
tion, it  is  concluded  that  their  orbits  are  but  slightly 
elliptical.     They  are  supposed  to   revolve  on  their  re- 
spective axes,  like  our  own   satellite,  the  moon,  once 
during  every  periodic  revolution. 

267.  As  these  orbits  lie  near  the  plane  of  the  ecliptic, 
they  have  to  pass  through  his  broad  shadow  when  in 
opposition  to  the  sun,  and  be  totally  eclipsed  at  every 
revolution.     To  this  there  is  but  one  exception.     As  the 
fourth  satellite  departs  about  3°  from  the  plane  of  Jupi- 
ter's orbit,  and  is  quite  distant,  it  sometimes  passes  above 
or  below  the  shadow,  and   escapes   eclipse.     But  such 
escapes  are  not  frequent. 

265.  How  are  their  orbits  situated  ?    How  satellites  appear  to  move  ? 

266.  Direction  of  secondaries  ?      Form  of   orbits  ?      How   ascertained  ? 
What  motion  on  axes  ? 

2<>7.  What  said  of  eclipses  ?  Of  fourth  satellite?  Of  solar  eclipses  upon 
Jupiter  ?  Number  of  solar  and  lunar  ? 


126 


ASTRONOMY. 


These  moons  are  not  only  often  eclipsed,  but  they  often 
eclipse  Jupiter,  by  throwing  their  owrn  dark  shadows 
upon  his  disk.  They  may  be  seen  like  dark  round  spots 
traversing  it  from  side  to  side,  causing,  wherever  that 
shadow  falls,  an  eclipse  of  the  sun.  Altogether,  about 
forty  of  these  eclipses  occur  in  the  system  of  Jupiter 
every  month. 

268.  The  immersions  and  emersions  of  Jupiter's  moons 
have  reference  to  the  phenomena  of  their  being  eclipsed. 
Their  entrance  into  the  shadow  is  the  immersion  •  and 
their  coming  out  of  it  the  emersion. 

ECLIPSES  OF  JUPITKK'S  MOONS,  EMKRSIONS,  ETC. 


^F 
\ 


4# 


1.  The  above  is  a  perpendicular  view  of  the  orbits  of  Jupiter's  satellites.     His  broau 
Bliadow  is  projected  in  a  direction  opposite  the  sun.    At  C,  the  second  satellite  is  suiter- 
ing  an  immersion,  and  will  soon  be  totally  eclipsed ;  while  at  D,  tlie  first  is  in  the  act  of 
emersion,  and  will  soon  appear  with  its  wonted  brightness.    The  other  satellites  are 
Been  to  cast  their  shadows  off  into  space,  and  are  ready  in  turn  to  eclipse  the  sun,  or  cut  off 
a  portion  of  his  beams  from  the  face  of  the  primary. 

2.  If  the  earth  were  at  A  in  the  cut,  the  immersion,  represented  at  C,  would  be  in- 
visible ;  and  if  at  B,  the  emersion  at  D  could  not  be  seen.    So,  also,  if  the  earth  wero 
exactly  at  F,  neither  could  be  seen  ;  as  Jupiter  and  all  his  attendants  would  be  directly 
beyond  the  sun,  and  would  be  hid  from  our  view. 

269.  The  system  of  Jupiter  may  be  regarded  as  a 
miniature  representation  of  the  solar  system,  and  as  fur- 
nishing triumphant  evidence  of  the  truth  of  the  Coper 
nican  theory.  It  may  also  be  regarded  as  a  great  natu- 
ral clock,  keeping  absolute  time  for  the  whole  world  ;  as 
the  immersions  and  emersions  of  his  satellites  furnish  a 
uniform  standard,  and,  like  a  vast  chronometer  hung  up 
in  the  heavens,  enable  the  mariner  to  determine  his  lon- 
gitude upon  the  trackless  deep. 

268.  What  are  the  immersions  and  emersions  of  Jupiter's  moons  ?    (Arc 
the  immersions  and  emersions  always  visible  from  the  earth?    Why  not  ? 
Illustrate.) 

269.  How  may  the  system  of  Jupiter  be  regarded  ?    What  use  mac's  of  in 
naviga  ion  ?    (lll-'strate  method.    Much  u^ed  ?) 


SATELLITES    OF    THE    EXTERIOR    PLANETS.  127 


By  l*m°  and  careful  observations  upon  these  satellites,  astronomers  have  been  al;'e 
to  construct  tables,  showing  the  exact  time  when  each  immersion  and  emersion  will  take 
place,  at  Greenwich  Observatory,  near  London.  Now  suppose  the  tables  fixed  the  time 
for  a  certain  satellite  to  be  eclipsed  at  12  o'clock  at  Greenwich,  but  we  find  it  to  occur  at 
9  o'clock,  for  instance,  by  our  local  time:  this  would  show  that  our  time  was  three- hours 
behind  the  time  at  Greenwich;  or,  in  other  words,  that  we  were  three  hours,  or  45°, 
tce»t  of  Greenwich.  If  our  time  was  ahead  of  Greenwich  time,  it  would  show  that  we 
were  efint  of  that  meridian,  to  the  amount  of  15°  for  every  hour  of  variation.  But  this 
method  of  finding  the  longitude  is  less  used  than  the  "  lunar  method"  (Art.  245),  on  ac- 
count of  the  greater  difficulty  of  making  the  necessary  observations. 

270.  By  observations  upon  the  eclipses  of  Jupiter's 
moons,  as  compared  with  the  tables  fixing  the  time  of 
their  occurrence,  it  was  discovered  that  light  had  a  pro- 
gressive motion,  at  the  rate  of  about  200,000  miles  per 
second. 

1.  This  discovery  may  be  illustrated  by  again  referring  to  the  opposite  cut    In  th* 
year  1675,  it  was  observed  by  Roemer,  a  Danish  astronomer,  that  when  the  earth  was 
nearest  to  Jupiter,  as  at  E,  the  eclipses  of  his  satellites  took  place  8  minutes  13  seconds 
foont>r  than  the  mean  time  of  the  tables ;  but  when  the  earth  was  farthest  from  Jupiter, 
as  at  F,  the  eclipses  took  place  8  minutes  and  13  seconds  later  than  the  tables  predicted 
tiie  entire  difference  being  16  minutes  and  26  seconds.     This  difference  of  time  he 
ascribed  to  the  progressive  motion  of  light,  which  he  concluded  required  16  minutes  and 
26  seconds  to  cross  the  earth's  orbit  from  E  to  F. 

2.  This  progress  may  be  demonstrated  as  follows: — 16m.  26s.  =  986s.    If  the  radius  of 
the  earth's  orbit  be  95  millions  of  miles,  the  diameter  must  be  twice  that,  or  190  mil- 
lions.   Divide  190,000,000  miles  by  986  seconds,  and  we  have  192,697$™  miles  as  the 
progress  of  light  in  each  second.     At  this  rate,  light  would  pass  nearly  eight  times 
around  the  glob«?  at  every  tick  of  the  clock,  or  nearly  500  times  every  minute  1 

SATURN. 

271.  The  moons  of  Saturn  are  eight  in  number,  and 
are  seen  only  with   telescopes  of  considerable  power. 
The  best  time  for  observ- 
ing  them    is  when    the 

planet  is  at  his  equinoxes, 
and  his  rings  are  nearly 
invisible. 

In  January,  1849,  the  author  saw  five 

of  these  satellites,  as  represented  in  the  adjoining  cnt  The  rings  appeared  only  nfl  a 
line  of  light,  extending  each  way  from  the  planet,  and  the  satellites  were  in  the  direction 
of  the  line,  at  different  distances,  as  here  represented. 

272.  These  satellites  all  revolve   eastward  with    the 
rings  of  the  planet,  in  orbits  nearly  circular,  and,  with 
the  exception  of  the  eighth,  in  the  plane  of  the  rings. 
Their  mean  distances,  respectively,  from  the  planet's  cen- 

2TO.  What  discovery  by  observing  these  eclipses  ?  (Illustrate  method. 
Diagram.  Demonstration.) 

271.  Number  of  Saturn's  moons  ?    How  seen  ?    Best  time  ? 

272.  How  revolve?      Shape    of   orbits?      How    situated?      Distances? 
Period*  ? 


SATELLITES   OF  SATURN. 


128  ASTRONOMY. 


tcr  are  from  123.000  to  2,366,000  miles;  and  their  pe- 
riods from  22  Lours  to  79  days,  according  to  their  dis- 
tances. 

The  distances  and  periods  of  the  satellites  of  Saturn  are  as  follows: 

Distance  in  miles.  Periodic  tiins.  Distance  in  miles.  Periodic  tmie. 


1st 118,000 0  day  22|  hours. 

2<1 152,000 1    "      9 

8d. 188,000 1    "    21        " 

4th 240,000 2    "    17        " 


5th 336.000 4  days  12  hours, 

6th ..778,000 15     "    22    " 

7th 940,000 22      •*•      0     " 

Sth....  2,268,000 70     u      1    " 


COMPAKATIVB  DISTANCES  OF  THE  MOONS  OF  SATURN. 

2. 

••••— — -— — 


273.  The  sixth  of  these  satellites  is  the  largest,  sup- 
posed to  be  about  the  size  of  Mercury;  and  the  remain- 
der grow  smaller  as  they  are  nearer  the  primary.     They 
are  seldom  eclipsed,  on  account  of  the  great  inclination 
of  their  orbits  to  the  ecliptic,  except  twice  in  thirty  years, 
when  the  rings  are  edgewise  toward  the  sun.    The  eighth 
satellite,  which  has  been  studied  more  than  all  the  rest, 
is  known  to  revolve  once  upon  its  axis  during  every 
periodic  revolution ;  from  which  it  is  inferred  that  they 
all  revolve  on  their  respective  axes  in  the  same  manner. 

1.  Let  the  line  A  B  represent  the 
plane  of   the  planet's  orbit,  C  D  his 
u-xis,  and   ¥  F  the  plane  of  his  rings. 
The  satellites  being  in  the  plane  of  the 
rings,  will  revolve  around  the  shadow 
of   the    primary,   instead    of  passing 
through  it,  and  being  eclipsed. 

2.  At  the  time  of  his  equinoxes,  how- 
ever, when  the  rings  are  turned  toward 
the  sun  (see  A  and  E,  cut,  page  92), 
they  must  be  in  the  center  of  the  shad- 
ow   on    the    opposite    side ;   and  the 

moons,  revolving  in  the  plane  of  the  rings,  must  pass  through  the  shadow  at  every 
revolution.  The  eighth,  however,  may  sometimes  escape,  on  account  of  Ms  departure 
from  the  plane  of  the  rings,  as  shown  in  the  cut 

URANUS. 

274.  Uranus  is  supposed  to  be  attended  by  six  secon- 
daries.    Sir  Win.  Herschel  recorded  that  he  saw  this 
number,  and  computed  their  periods  and  distances  ;  and 
on  his  authority  the  opinion  is  generally  received,  though 

273.  Size?    Eclipses  of ?    When?    Why  not  oftener  ?    (Illustrate.) 

274.  Satellites  of  Uranus  ?    Upon  what  authority  ?    Distances?    Periods? 
Situation  of  orbits?     Form?     Direction  in  revolution  2    Remark  of  Dr. 
Kerschel  \ 


NATURE     AND     CAUSE     OF     TIDES.  129 


no  other  observer  Las  ever  been  able  to  discover  mori 
than  three.  They  are  situated  at  various  distances,  and 
revolve  in  from  1  day  and  21  hours  to  117  days.  Their 
orbits  are  nearly  perpendicular  to  the  ecliptic,  and  they 
revolve  backward,  or  from  east  to  west,  contrary  to  all 
the  other  motions  of  our  planetary  system.  Their  or- 
bits are  nearly  circular,  and  they  are  described  by  Dr. 
Herschel  as  "  the  most  difficult  objects  to  obtain  a  sight 
of,  of  any  in  our  system." 

The  distances  and  periods  of  the  system  of  Uranus,  as  laid  down  by  Dr.  Herschel,  are 
as  follows : 


Distance  in  miles.  Periodic  times. 

1  st 224,000 Iday  21  hours. 

2d 296,000 8    "    17      " 

3d 340,000 10    "   23      " 


Distance  in  miles.  Periodic  times. 

4th 890,000 13  days  11  hours. 

5th.... '..777,000 38    "      2      " 

6th....  1,556,000 117    "    17      " 


NEPTUNE. 

275.  Neptune  is  known  to  be  attended  by  one  satel- 
lite, and  suspected  of  having  two.  Professor  Bond,  of 
Cambridge,  Mass.,  states  that  he  has  at  times  been  quite 
confident  of  seeing  a  second.  The  mean  distance  of  the 
known  satellite  from  its  primary  is  236,000  miles,  or  near 
the  distance  of  our  moon.  Its  period  is  only  5  days  and 
21  hours. 

"We  have  here  another  illustration  of  the  great.law  of  planetary  motion  explained  nt 
74  So  great  is  the  attractive  power  of  Neptune,  that  to  keep  a  satellite,  at  the  distance 
of  our  moon,  from  falling  to  his  surface,  it  must  revolve  some  five  times  as  swiftly  as  si; o 
revolves  around  the  earth.  The  centripetal  and  centrifugal  forces  must  be  balanced  in 
all  cases,  as  the  laws  of  gravitation  and  planetary  motion,  discovered  by  Newton  aud 
Kepler,  extend  to  and  prevail  among  all  the  secondaries. 


CHAPTER    VIII. 

NATURE     AND     CAUSE     OP     TIDES. 

276.  TIDES  are  the  alternate  rising  and  falling  of  the 
waters  of  the  ocean,  at  regular  intervals.  Flood  tide  is 
when  the  waters  are  rising ;  and  eUb  tide,  when  they  are 


275.  What  said  of  Neptune's  secondaries?    Eemark  of  Prof.  Bond  ?    Dis- 
nce  and  period  of  th< 

276.  What  are  tides  \ 
lo  they  ebb  und  flow  ? 


tance  and  period  of  the  Known  satellite  ?    (Remark  in  note.) 
276.  What  are  tides  ?    Flood  and  ebb  tides  ?    High  and  low?    How  cften. 


130  ASTRONOMY. 


falling.  The  highest  and  lowest  points  to  which  they 
go  are  called,  respectively,  high  and  low  tides.  The 
tides  ebb  and  flow  twice  every  twenty-four  hours — i.  £., 
we  have  two  flood  and  two  ebb  tides  in  that  time. 

277.  The  tides  are  not  uniform,  either  as  to  time  or 
amount.     They  occur  about  50  minutes  later  every  day 
(as  we  shall  explain  hereafter),  and  sometimes  rise  much 
higher  and  sink  much  lower  than  the  average.     These 
extraordinary  high  and  low  tides  are  called,  respectively, 
spring  and  neap  tides. 

278.  The  cause  of  the  tides  is  the  attraction  of  the  sun 
and  moon  upon  the  waters  of  the  ocean.     But  for  this 
foreign  influence,  as  we  may  call  it,  the  waters  having 
found  their  proper  level,  would  cease  to  heave  and  swell, 
as  they  now  do,  from  ocean  to  ocean,  and 

would  remain  calm  and  undisturbed,  save 
by  its  own  inhabitants  and  the  winds  of 
heaven,  from  age  to  age. 

In  this  figure,  the  earth  is  represented  as  surrounded  by  water,  in  a 
stiite  of  rest  or  equilibrium,  as  it  would  be  were  it  not  acted  upon  by 
the  sun  and  moon. 

279.  To  most  minds,  it  would  seem  that  the  natural 
effect  of  the  moon's  attraction  would  be  to  produce  a 
single  tide-wave  on  the  side  of  the  earth  toward  the 
moon.     It  is  easy,  therefore,  for  students  to  conceive  how 
the  moon  can  produce  one  flood  and  one  ebb 

tide  in  twenty-four  hours. 

1.  In  this  cut,  the  moon  is  shown  at  a  distance  above  the  earth, 
and  attracting  the  waters  of  the  ocean,  so  as  to  produce  a  high  tide 
at  A.      But  as  the  moon  makes  her  apparent  westward  revolution 
around  the  earth  but  once  a  day,  the  simple  raising  of  a  flood  tide 
on  the  side  of  the  earth  toward  the  moon,  would  give  us  but  one  flood 
and  one  ebb  tide  in  twenty-four  hours ;  whereas  it  is  known  that  we 
have  two  of  each. 

2.  "  The  tides,"  says  Dr.  Herschel,  "  are  a  subject  on  which  many 
persons  find  a  strange  difficulty  of  conception.    That  the  moon,  by  her 
attraction,  should  heap  up  the  waters  of  the  ocean  under  her,  seems 
to  many  persons  very  natural.    That  the  same  cause  should,  at  the 

same  time,  heap  them  up  on  the  opposite  side  of  the  earth  (viz.,  at  B  in  the  figure),  seems 
to  many  palpably  absurd.  Yet  nothing  is  more  true." 

280.  Instead  of  a  single  tide-wave  upon  the  waters  of 

277.  Are  the  tides  uniform?    What  variation  of  time  ?    As  to  amount? 
What  are  these  extraordinary  high  and  low  tides  called  ? 

278.  The  cause  of  tides  ?    How  but  for  this  influence  ? 

279.  What  most  obvious  effect  of  the  moon'a  attraction  ?    (Substance  ot 
note  1  ?    Remark  of  Dr.  Herschel  ?) 


NATURE      A.ND     CAUSE     OF     TIDES.  131 


the  globe,  directly  under  the  moon,  it  is  found  that  on 
the  side  of  the  earth  directly  opposite  there  is  another 
high  tide  ;  and  that  half  way  between  these  two  high 
tides  are  two  low  tides.  These  four  tides,  Two  TIDE.WAVE(i- 
viz.,  two  high  and  two  low,  traverse  the 
ocean  from  east  to  west  every  day,  which 
accounts  for  both  a  flood  and  an  ebb  tide 
every  twelve  hours. 

In  this  cut,  we  have  a  representation  of  the  tide-waves  as  they 
actually  exist,  except  that  their  hight,  as  compared  with  the  magni- 
tude of  the  earth,  is  vastly  too  great  It  is  designedly  exaggerated, 
the  better  to  illustrate  the  principle  under  consideration.  While 
the  moon  at  A  attracts  the  waters  of  the  ocean,  and  produces  a  high 
tide  at  B,  we  see  another  high  tide  at  C  on  the  opposite  side  of  the 
globe.  At  the  same  time  it  is  low  tide  at  D  and  E. 

281.  The  principal  cause  of  the  tide-wave  on  the  side 
of  the  earth  opposite  the  moon  is  the  difference  of  the 
moon's  attraction  on  different  sides  of  the  earth. 

If  the  student  well  understands  the  subject  of  gravitation  (65),  he  will  easily  perceive 
how  a  difference  of  attraction,  as  above  described,  would  tend  to  produce  an  elongation 
of  the  huge  drop  of  water  called  the  earth.  The  diameter  of  the  earth  amounts  to  about 
7j\,th  of  the  moon's  distance;  so  that,  by  the  rule  (69;,  the  difference  in  her  attraction 
on  the  side  of  the  earth  toward  her,  and  the  opposite  side,  would  be  about  -,-^th.  The 
attraction  being  stronger  at  B  {in  the  last  cut)  than  at  the  earth's  center,  and  stronger  at 
her  center  than  at  C.  would  tend  to  separate  these  three  portions  of  the  globe,  giving 
the  waters  an  elongated  form,  and  producing  two  opposite  tide-waves,  as  shown  in  the 
Hit 

282.  A  secondary  cause  of  the  tide-wave  on  the  side 
of  the  earth  opposite  the  moon,  is  the  revolution  of  the 
earth  around  the  common  center  of  gravity  between  the 
earth  and  rnoon,  thereby  generating  an  increased  centri- 
fugal force  on  that  side  of  the  earth. 

The  center  of  gravity  between  the  earth  and  moon  is  the  point  where  they  would 
exactly  balance  each  other,  if  connected  by  a  rod,  and  poised  upon  a  fulcrum. 

CENTEB  OF  GRAVITY  BETWEEN  THE  EARTH  AND  MOON. 

Moon 


This  point,  which,  according  to  Ferguson,  is  about  6,000  miles  from  the  earth's  center 
Is  represented  at  A  in  the  above,  and  also  in  the  next  cut 

280.  How  many  tide-waves  are  there  on  the  globe,  and  how  situated  ? 

281.  State  the  principal  cause  of  the  wave  opposite  the  moon  1    (Demon- 
btrate  by  diagram.) 

282.  What  other  cause  operates  with  the  one  just  stated  to  produce  tho 
tide-wave  opposite  the  moon  ?    (What  is  the  center  of  gravity  between  the 
earth  and  the  moon  ?    Where  is  it  situated  ?    Illustrate  the  operation  of  thia 
secondary  cause.    Diagram.) 


132  ASTRONOMY. 


/SECONDARY   CAUSE   OF  HIGH   TIDE   OPPOSITE  THE  MOON. 


1.  The  point  A  represents  the  center  of  gravity  between  the  ear  h  and  moon  ;  and  aa 
is  this  point  which  traces  the  regular  curve  of  the  earth's  orbit,  it  is  represented  in 

the  arc  of  that  orbit,  while  the  earth's  center  is  6,000  miles  one  side  of  it.  Now  the  law 
of  gravitation  requires  that  while  both  the  moon  and  earth  revolve  around  the  sun,  they 
should  also  revolve  around  the  common  center  of  gravity  between  them,  or  around  the 
point  A.  This  would  give  the  earth  a  third  involution,  in  addition  to  that  around  the 
sun  and  on  her  axis.  The  small  circles  show  her  path  around  the  center  ot  gravity, 
and  the  arrows  her  direction. 

2.  This  motion  of  the  earth  would  slightly  increase  the  centrifugal  tendency  at  B, 
and  thus  help  to  raise  the  tide-wave  opposite  the  moon.     But  as  this  motion  is  slow, 
corresponding  with  the  revolution  of  the  moon  around  the  earth,  the  centrifugal  fored 
could  not  be  greatly  augmented  by  such  a  cause. 

283.  As  the  moon,  which  is  the  principal  cause  of  the 
tides,  is  revolving  eastward,  and  comes  to  the  meridian 
later  and  later  every  night,  so  the  tides  are  about  50 
minutes  later  each  successive  day.     This  makes  the  in- 
terval between  two  successive  high  tides  12  hours  and  25 
minutes.  Besides  this  daily  lagging 

with  the  moon,  the  highest  point    ™E-WAVES  BEHIND  TnE  MOON. 
of  the  tide-wave  is  found  to   be 
about  45°  behind  or  east  of  the 
moon,  so  that  high  tide  does  not   /' 
occur  till  about  three  hours  after 
the  moon  has  crossed  the  merid- 
ian.    The  waters  do  not  at  once 
yield  to  the  impulse  of  the  moon's 
attraction,   but   continue   to   rise 
after  she  has  passed  over. 

In  the  cut,  the  moon  is  on  the  meridian,  but  the  highest  point  of  the  wave  is  at  A,  01 
46°  east  of  the  meridian ;  and  the  corresponding  wave  on  the  opposite  side  at  B  is 
equally  behind. 

284.  The  time  and  character  of  the  tides  are  also 
affected  by  winds,  and  by  the  situation  of  different  places. 
Strong  winds  may  either  retard  or  hasten  the  tides,  or 
may  increase  or  diminish  their  hight ;  and  if  a  place  is 
situated  on  a  large  bay,  with  but  a  narrow  opening  into 
the  sea,  the  tide  will  be  longer  in  rising,  as  the  bay  has 

283.  What  daily  lagging  of  the  tides  ?  Interval  between  two  successive 
hig \\  tides  I  What  other  lagging  ?  Cause  of  this  last  $ 

264.  What  modification  of  the  time  and  character  of  the  tides  ? 


NATURE     AND     CAUSE     OF     TIDES. 


133 


to  fill  up  through  a  narrow  gate.  Hence  it  is  not  usually 
high  tide  at  New  York  till  eight  or  nine  hours  after  the 
moon  has  passed  the  meridian.  . 

285.  As  botH  the  sun  and  moon  ;are  concerned  in  the 
production  of  tides,  and  yet  arer  constantly  changing 
their  positions  with  respect  to  the  earth  and  to  each 
other,  it  follows  that  I  they  /sometimes  act  against  each 
others  and  measurably"i*eutralize  each  other's  influence  ; 
while  at  other  times  iiiQjicotnbine  their  forces,  and  mutu- 
ally assist  each  other,  in  the  latter  case,  an  unusually 
high  tide  occurs,  called  the  Spring  Tide.  This  happens 
both  at  new  and  full  moo3| 


CAUSE  OF  8PBING  TIDES. 


1.  Here  tlie  sun  and  moon,  being  in  conjunction,  unite  their  forces  to  produce  an  ex- 
traordinary tide.    The  same  effect  follows  when  they  are  in  opposition  ;  so  that  \ve  have 
two  spring  tides  every  month — namely,  at  new  and  full  moon. 

2.  If  the  tide-waves  at  A  and  B  are  one-third  higher  at  the  moon's  quadrature  than 
usual,  those  of  C  and  D  will  be  one-third  lower  than  usual. 

286.  Although  the  sun  attracts  the  earth  much  more 
powerfully,  as  a  whole,  than  the  moon  does,  still  the 
moon  contributes  more  than  the  sun  to  the  production 
of  tides.  Their  relative  influence  is  as  one  to  three. 
The  nearness  of  the  moon  makes  the  difference  of  her 
attraction  on  different  sides  of  the  earth  much  greater 
than  the  difference  of  the  sun's  attraction  on  different 
sides. 

It  must  not  be  forgotten  that  the  tides  are  the  result  not  so  much  of  the  attraction  of 
the  sun  and  moon,  as  a  whole,  as  of  the  difference  in  their  attraction  on  different  sides 

235.  Do  the  sun  and  moon  always  act  together  in  attracting1  the  watere  I 
"Why  not?  How  affect  each  other's'  influence  ?  Effect  on  the  tides  ?  What 
ure  Xpi-ing  Tiles  ?  When  do  they  occur  ?  (Illustrate  by  diagram  the  cause 
of  spring  tide,  when  the  sun  and  moon  are  in  conjunction.) 

286.  Comparative  influence  of  sun  and  moon  in  the  production  of  tides  ? 
Why  inoon'»  influence  the  greatest  ?  (Substance  of  note  ?  Demonstration.? 

12 


134: 


ASTRONOMY. 


SPKLNG  AND   NEAP  TIDES. 


of  the  earth,  caused  by  a  difference  in  the  dintnnces  of  the  several  parts.  The  attrr.'v 
tion  being  inversely,  as  the  square  of  the  distance  (69  ,  the  influence  of  the  sun  and 
moon,  respectively,  must  be  in  the  ratio  of  the  earth's  diameter  to  their  distances.  Now 
the  difference  in  the  distance  of  two  sides  of  the  earth  from  the  moon  is  jA  th  of  tho 
moon's  distance ;  as  240,000  -J-  8,000  =  30 ;  while  the  difference,  as  compared  with  tho 
distance  of  the  sun,  is  only  -pf^^th,  as  95,000,000  -f-  8,000  =  11,875. 

/^  287.  \When  the  moon  is  in  quadrature^  and  her  influ- 

/  ence  is  partly  neutralized  by  the  sun,  which  now  acts 

against  her,  the  result  is  a  very  low  tide,  called  Neap 

\   Tide. 

\  The  whole  philosophy  of  spring 
hnd  neap  tides  may  be  illustrated  by 
the  annexed  diagram. 

1.  On  the  right  side  of  the  cut,  the 
Bun  and  moon  are  in  conJimction, 
and  unite  to  produce  a  spring  tide. 

2.  At   the  first  quarter,  their   at- 
traction acts  at  right  angles,  and  the 
sun.  instead  of  contributing  to  the 
lunar  tide-waves,  detracts  from  it  to 
the  amount    of  his   own   attractive 
force.    The  tendency  to  form  a  tide 
of  his  own,  as    represented    in    the 
figure,  reduces  the  moon's  wave  to 
the  amount  of  one-third. 

3.  At  the  full  moon,  she  is  in  oppo- 
sition to  the  sun,  and  their  joint  at- 
traction  acting  again   in    the    same 
line,  tends  to  elongate  the  fluid  por- 
tion of  the  earth,  and  a  second  spring 
tide  is  produced. 

4.  Finally,  at    the    third  quarter, 
the  sun  and  moon  act  against  each 

other  again,  and  the  second  neap  tide  is  the  result.  Thus  we  have  two  spring  and  two 
neap  tides  during  every  lunation — the  former  at  the  moon's  syzygies,  and  the  latter  at  '.ier 
quadratures. 

288.  The  tides  are  subject  to  another  periodic  varia- 
tion, caused  by  the  declination  of  the  sun  and  moon 

north  and  SOUth  of  the  equator.       As    TIDES  AFFECTED  BY  DECLINA- 

the  tendency  of  the  tide-wave  is  to 
rise  directly  under  the  sun  and 
moon,  when  they  are  in  the  south, 
as  in  winter,  or  in  the  north,  as  in 
summer,  every  alternate  tide  is 
higher  than  the  intermediate  one. 

At  the  time  of  the  equinoxes,  the  sun  being  over  the 
equator,  and  the  moon  within  5£°  of  it,  the  crest  of  the 
great  tide-ware  will  be  on  the  equator ;  but  as  the  sun 
and  moon  decline  south  to  A,  one  tide-wave  forms  in 
the  south,  as  at  B,  and  the  opposite  one  in  the  north,  as  at  C.  If  the  declination  wan 
nort/i,  as  shown  at  D,  the  order  of  the  tides  would  be  reversed.  The  following  diagram, 

-287.  What  are  Neap  Tides  f    Their  cause?    (Illustrate  entire  philosophy 
by  diagram.) 

'.88. "What  other  periodic  variations  mentioned?     (Explain  cause,  and 
illustrate,) 


NATURE     AND     CAUSE     OF     TIDES. 


135 


if  carefully  studied,  will  more  fully  illustrate  the  subject  of  the  alternate  high  and  low 
tides,  in  high  latitudes,  in  winter  and  summer: 


ALTERNATE   HIGH   AND   LOW   TIDES. 


1.  Let  the  line  A  A  represent  the  plane  of  the  ecliptic,  and  B  B  the  equinoctial.    On 
the  21st  of  June,  the  day  tide-wave  is  north,  and  the  evening  wave  south,  so  that  tho 
tide  following  about  three  hours  after  the  sun  and  moon  will  be  higher  than  the  inter- 
mediate one  at  3  o'clock  in  the  morning. 

2.  On  the  23d  of  December,  the  sun  and  moon  being  over  the  southern  tropic,  the 
highest  wave  in  the  southern  hemisphere  will  be  about  3  o'clock  P.  M.,  and  the  lowest 
about  3  o'clock  A.  M. ;  while  at  the  north,  this  order  will  be  reversed.    It  is  on  this  ac- 
count that  in  high  latitudes  every  alternate  tide  is  higher  than  the  intermediate  ones; 
the  evening  tides  in  summer  exceeding  the  morning  tides,  and  the  morning  tides  in  win- 
ter exceeding  those  of  evening. 

289.  All  spring  and  neap  tides  are  not  alike  as  to  their 
elevation  and  depression.  As  the  distances  of  the  sun 
and  moon  are  varied,  so  are  the  tides  varied,  especially 
by  the  variations  of  the  moon. 

VARIATIONS  IN  THE  SPRING  TIDES. 


1.  At  A,  the  earth  is  in  aphelion,  and  the  moon  in  apogee.  As  both  the  sun  and  moon 
are  at  their  greatest  distances,  the  earth  is  least  affected  by  their  attraction,  and  the  spring 
tides  are  proportionately  low. 

2.  At  B,  the  earth  is  in  perihelion,  and  the  moon  in  perigee  ;  so  that  both  the  sun  and 
moon  exert  their  greatest  influence  upon  our  globe,  and  the  spring  tides  are  highest,  as 
shown  in  the  figure.     In  both  cases,  the  sun  and  moon  are  in  conjunction,  but  the  varia- 
tion in  the  distances  of  the  sun  and  moon  causes  variations  in  the  spring  tides. 

290.  In  the  open  ocean,  especially  the  Pacific,  the  tide 
rises  and  falls  but  a  few  feet ;  but  when  pressed  into  nar- 
row bays  or  channels,  it  rises  much  higher  than  under 
ordinary  circumstances. 


289  Are  all  spring  and  neap  tides  alike  ?  By  what  are  they  modified  ? 
(Illustrate  by  diagram.) 

290.  Hight  of  tides  in  open  seas  ?  How  in  narrow  bays  and  channels  ? 
fllight  at  different  points  on  our  coast,  ?) 


136  ASTRONOMY. 


The  average  ele   ition  of  the  tide  at  seveiai  points  on  on    '.r^t  is  .is  follows: 

Cumberland,  head  of  the  Bay  of  Fundy 71  feet. 

Boston 114  " 

New  Haven 8    " 

Now  York 5    " 

Charleston,  8.  0. . 6    " 

291.  As  the  great  tide-waves  proceed  from  east  to 
west,  they  are  arrested  by  the  continents,  so  that  the 
waters  are  permanently  higher  on  their  east  than  on  their 
west  sides.  The  Gulf  of  Mexico  is  20  feet  higher  than 
the  Pacific  Ocean,  on  the  other  side  of  the  Isthmus  ;  and 
the  Red  Sea  is  30  feet  higher  than  the  Mediterranean. 
Inland  seas  and  lakes  have  no  perceptible  tides,  because 
they  are  too  small,  compared  with  the  whole  surface  of 
the  globe,  to  be  sensibly  affected  by  the  attraction  of  the 
sun  and  moon. 

We  have  thus  stated  the  principal  facts  connected  with 
this  complicated  phenomenon,  and  the  causes  to  which 
they  are  generally  attributed.  And  yet  it  is  not  certain 
that  the  philosophy  of  tides  is  to  this  day  fully  under- 
stood. La  Place,  the  great  French  mathematician  and 
astronomer,  pronounced  it  one  of  the  most  difficult  prob- 
lems in  the  whole  range  of  celestial  mechanics.  It  is 
probable  that  the  atmosphere  of  our  globe  has  its  tides, 
as  well  as  the  waters  ;  but  we  have  no  means,  as  yet,  for 
definitely  ascertaining  the  fact. 


CHAPTER    IX. 

OF     COMETS. 

292.  COMETS  are  a  singular  class  of  bodies,  belonging 
to  the  solar  system,  distinguished  for  their  long  trains  of 
light,  their  various  shapes,  and  the  great  eccentricity  ot 
their  orbits.  Their  name  is  from  the  Greek  coma,  which 

291.  Direction  of  tide-waves  ?     What  result  ?    Instances  cited  ?    Have  in- 
land seas  and  lakes  anv  tides  ?     Why  not  ?     Remarks  respecting  philosophy 
of  tides  ?    Of  La  Place  ?    Atmospheric  tides  ? 

292.  What  are  comets?    Derivation  of  name  ?    Are  they  opake  or  sclf- 
luHiirfbus? 


OF    COMETS. 


GREAT  COMET  OP  871  BEFORE  CHRIST. 


signifies  heard  or  Jiair,  on  account  of  their  bearded  or 
hairy  appearance.  They  are  known  to  be  opake,  from 
the  iact  that  they  sometimes  exhibit  phases,  which  show 
that  they  shine  only  by  reflection. 

293.  Cornets  usually  consist  of  three  parts — the  nn- 
cleus,  the  envelope,  and  the  tail.     The  nucleus  is  what 
may  be  called  the  ~body  or  head  of   the   comet.      Thy 
envelope,  is   the   nebulous  or  hairy  covering   that   sur- 
rounds the  nucleus ;  and 

the  tail  is  the  expan- 
sion or  elongation  of  the 
envelope.  But  all  comets 
have  not  these  parts. 
Some  have  no  percept- 
ible nucleus ;  their  entire 
structure  being  like  that 
of  a  thin  vapory  cloud 
passing  through  the  dis- 
tant heavens.  Others 
have  but  a  slight  envelope 
around  a  strongly  marked 
nucleus. 

The  great  comet  that  appeared  371 

years  before  Christ  exhibited  the  different  parts  of  a  comet  with  great  distinctness;  on 
which  account,  as  well  as  for  its  striking  magnificence,  we  give  a  view  of  it  in  the  above 
cut 

294.  The  tails  of  comets  usually  lie  in    a  direction 
opposite  to  the  sun,  so  that  from  perihelion  to  aphelion 
they  precede  their  nuclei  or  heads ;  or,  in  other  words, 
comets  seem,  after  having  passed  their  perihelion,  to  back 
out  of  the  solar  system.     Their  tails  are  usually  curved 
more  or  less,  being  concave  toward  the  region   from 
whence  they  come.     This  is  well  shown  in  the  comets  of 
1811,  1843,  and  in  the  following  cut.     That  of  1689  is 
said  to  have  been  curved  like  a  Turkish  sabre.     The 
cause  of  this  curvature  of  the  tails  of  comets  is  supposed 
to  be  a  very  rare  ethereal  substance  which  pervades 

293.  Parts  of  a  comet?    Describe  each.    Hove  all  cornets  these  three  parte  ? 
(What  comet  shown  as  a  sample  in  the  cut  ?) 

294.  Direction  of  the  tails  of  comets  2    How  curved '«    Cause  of  this  cur- 
vature ? 


138  ASTRONOMY. 


space,  and  offers  a  slight  resistance  to  their  progress.  Of 
course  it  munt  be  almost  infinitely  attenuated,  as  the 
comets  themselves  are  a  mere  vapor,  which  could  make 
no  progress  through  the  spaces  of  the  heavens,  were  they 
not  very  nearly  a  vacuum.  They  could  no  more  pass  u 
medium  as  dense  as  our  atmosphere,  than  an  ordinary 
cloud  could  pass  through  the  waters  of  the  sea. 

295.  The  form  of  the  comets'  orbits  is  generally  that 
of  an  ellipse  greatly  flattened  or  elongated.  The  sun 
being  near  one  end  of  the  ellipse,  and  the  planets  com- 
paratively in  his  immediate  neighborhood,  the  comets 
are  in  the  vicinity  of  the  sun  and  planets  but  a  short 
time,  and  then  hasten  outward  again  beyond  the  limits  of 
human  vision,  with  the  aid  of  the  best  telescopes,  to  be 
sjone  again  for  centuries. 


ORBIT   OP   A   GOMKT. 


Here  it  will  be  seen  that  the  orbit  is  very  eccentric,  that  the  perihelion  point  is  very 
near  the  sun,  and  the  aphelion  point  very  remote. 

296.  The  tails  of  comets  do  not  continue  of  the  same 
uniform  length.  They  increase  both  in  length  and 
breadth  as  they  approach  the  sun,  and  contract  as  they 
recede  from  him,  until  they  often  nearly  disappear  before 
the  comet  gets  out  of  sight.  Instances  have  occurred  in 
which  tails  of  comets  have  been  suddenly  expanded  or 
elongated  to  a  great  distance.  This  is  said  to  have  been 
the  case  with  the  great  comet  of  1811. 

295.  Form  of  the  orbits  of  comets  ?    What  near  the  earth  but  little  of  'Jie 
tame? 

296.  What  said  of  the  contraction  and  expansion  of  the  tails  of  cort?ots 
What  specimen  shown  in  the  cut  I 


OF   COMETS. 


139 


GREAT  COMET   OF   1S11. 


297.  Comets  have  been  known  to  exhibit  several  tails 
at  the  same  time.     That  of  1744,  represented  in  the  cut, 
had   no    less    than   six   tails 

spread  out  in  the  heavens, 
like  an  enormous  fan.  The 
comet  of  1823  is  said  to  have 
had  two  tails,  one  of  which 
extended  toward  the  sun. 

The  comet  of  1744,  represented  in  this  cut, 
excited  great  attention  and  interest.  It  ex- 
hibited no  train  till  within  the  distance  of  the 
orbit  of  Mars  from  the  sun ;  but  early  in 
March  it  appeared  with  a  tail  divided  into  six 
branches,  all  diverging,  but  curved  in  the 
same  direction.  Each  of  these  tails  was  about 
4°  wide,  and  from  30°  to  44°  in  length.  The 
edges  were  bright  and  decided,  the  middle 
faint,  and  the  intervening  spaces  as  dark  as 
the  rest  of  the  firmament,  the  stars  shining 
in  them.  When  circumstances  were  favor- 
able to  the  display  of  this  remarkable  body, 
the  scene  was  striking  and  magnificent,  al- 
most beyond  description. 

298.  The  heads  or  nuclei  of  comets  are  comparatively 
small.     The  following  table  shows  the  estimated  diam- 
eter in  five  different  instances  : 


The  comet 

of  1778     .    diameter 

of  head    33  miles. 

a 

1805     . 

a 

(C 

36 

u 

(C 

1799     . 

(C 

U 

462 

U 

U 

1807     . 

u 

u 

666 

u 

u 

1811     . 

u 

u 

428 

u 

9.97.  Have  they  ever  more  than  one  tail?    What  peculiarity  of  the  comet 
of  "323  ?    (What  specimen  ^  comet  with  several  tails  ?    Describe.) 


110 


ASTRONOMY. 


COMET  OF  1585. 


Many  comets  have  simply  the  envelope,  without  any  tail 
or  elongation.  Such  were  those  that  appeared  in  1585 
and  1763,  the  former  of  which  is 
represented  in  the  adjoining  cut, 
Cassini  describes  the  comet  of 
1682  as  being  as  round  and  as 
bright  as  Jupiter,  without  even  an 
envelope.  But  these  are  very  rare 
exceptions  to  the  general  charac- 
ter of  cometary  bodies. 

299.  The  tails  of  comets  are 
often  of  enormous  length  and 
magnitude.  That  of  371  before 
Christ  was  60°  long,  covering  one-third  of  the  visible 
heavens.  In  1618,  a  comet  appeared,  which  was  101° 
in  length.  Its  tail  had  not  all  risen  when  its  head 
reached  the  middle  of  the  heavens.  That  of  1680  had 
a  tail  70°  long ;  so  that  though  its  head  set  soon  after 
Rundown,  its  tail  continued  visible  all  night. 

GREAT  COMET  OF  1843. 


The  following  table  will  show  the  lens^h  of  the  tails  of  some  of  the  most  remarkable 
comets,  both  in  degrees  and  in  miles.  They  will  be  characterized  only  by  the  year  when 
they  appeared : 

Tieg.  Miles. 

80  85.000,006 

48,000.000 


Desr.  Miles. 

B.C.    371 60  140,000,000 

A.  D.  1456 60  70,000,000 

"     1618 104 65,000,000 

"     1680 70  123.000,000 

"     1689 68  100,000,000 


A.  D.I  744 
"     1769 

"     1811 23  132.000,000 

"     1843 60 130,000,000 


298.  What  of  the  size^  of  the  nuclei  of  comets  ?    Give  a  few  examples. 
What  comets  without  tails  ?    What  specimen  hi  the  cut?    What  said  ot  the 
comet  of  1682  ?     Are  such  comets  numerous  ? 

299.  What  of  the  size  of  the  tails  of  comets  ?    That  of  871  B.  C.  ?    Of 
A.  D.  1618  ?    Of  1680?    (What  specimen  in  cut,  and  its  length?    State  tho 
length  of  some  others  in  miles.) 


OF   COMETS.  141 


300.  The  velocity  with  which  comets  often  move  is 
truly  wonderful.     Their  motions  are  accelerated  as  they 
approach,  and  retarded  as  they  recede  from  the  sun ;  so 
that  their  velocity  is  greatest  while  passing  their  peri- 
helioris.      The  comet  of  1472  described  an  arc  of  the 
heavens   of  120°  in   extent  in   a  single  day !     That  of 
1680  moved,  when  near  its  perihelion,  at  the  rate  of 
1,000,000  miles  per  hour. 

301.  The  temperature  of  some  comets,  when  nearest 
the  sun,  must  be  very  great.     That  of  1680  came  within 
130,000  miles  of  the  sun's  surface,  and  must  have  re- 
ceived 28,000  times  the  light  and  heat  which  the  earth 
receives  from  the  sun — a  heat  more  than  2,000  times 
as  great   as  that  of  red-hot  iron  !     What  substance  can 
a  comet  be  composed  of  to  endure  the  extremes  of  heat 
and  cold  to  which  it  is  subject  ?     Some  have  supposed 
that  their  tails  were  caused  by  the  sun's  light  and  heat 
rarefying  and  driving  back  the  vapory  substance  com- 
posing the  envelope. 

302.  The  periods  of  but  few  comets  are  known.  That  of 
1818,  called  Enckds  Co- 
met, lias  a  period  of  only 

3  J  yeaw.    JBiclds  Comet 

lias  a  period  of  6f  years. 

That  of  1862  (then  first 

noticed  with  care,  and 

identified  as  the   same 

that   had   appeared   in 

1456,  1531,   and   1607) 

has  a  period  of  about  76 

years.     It  is  called  Ilal- 

Uy^s    Comet,   after    Dr. 

Halley,  who  determined 

its  periodic  time.     The 

great  comet  of  1680  has  a  periodic  time  of  570  years, 

so  that  its  next  return  to  our  system  will  be  in  the  year 

300.  Velocity  of  comets  ?    Uniform  or  not  ?    Comet  of  1472  ?    Of  1680  ? 
801.   Temperature?    Comet  of  1680?     Supposed  cause  of  their  tails  ? 
302.  Periods?    Encke's  ?    Biela's  ?    Halley's  ?    That  of  1680?    Supposed 
periods  of  ot!/  ers  ?    Opinions  of  Prof.  Nichol  and  Dr.  Herschel  ? 


14:2 


2250.  Many  are  supposed  to  have  periods  of  thousands 
of  years ;  and  some  have  their  orbits  so  modified  by  the 
attraction  of  the  planets,  as  to  pass  off  in  parabolic  curves, 
to  return  to  our  system  no  more. 

Prof.  Nichol  is  of  opinion  that  the  greater  number  visit  our  system  but  once,  and  then 
fly  off  in  nearly  straight  lines  till  they  pass  the  center  of  attraction  between  the  solar 
system  and  the  fixed  stars,  and  go  to  revolve  around  other  suns  in  the  far  distant  heav- 
ens. Sir  John  Herschel  expresses  the  same  opinion. 

303.  The  distances  to  which  those  comets  that  return 
must  go,  to  be  so  long  absent,  must  be  very  great.  Still 
their  bounds  are  set  by  the  great  law  of  gravitation,  for 
were  they  to  pass  the  point  "  where  gravitation  turns  the 
other  way,"  they  would  never  return.  But  some,  at 
least,  do  return,  after  their  "  long  travel  of  a  thousand 
years."  "What  a  sublime  conception  this  affords  us  of 
the  almost  infinite  space  between  the  solar  system  and 
the  fixed  stars. 

ORBIT  or  HALLEY'S  COMET. 


304:.  The  perihelion  distances  of  the  various  comets 
that  have  appeared,  and  whose  elements  have  been  esti- 
mated by  astronomers,  are  also  exceedingly  variable. 
While  some  pass  very  near  the  sun,  others  are  at  an  im- 
mense distance  from  him,  even  at  their  perihelion.  Of 
137  that  have  been  particularly  noticed, 

30  passed  between  the  sun  and  the  orbit  of  Mercury. 
44  between  the  orbits  of  Mercury  and  Yenus. 
34:         "  "          Yenus  and  the  earth. 

23  «          the  earth  and  Mars. 

6         "  "          Mars  and  Jupiter. 

803.  Distances  to  which  they  go  ?    Remark  respecting  the  law  of  gravi- 
uition  ?    What  specimen  of  orbit  given  ? 
304.  What  said  of  perihelion  distances  ?     How  many  noticed  ?    Where  dij 


OF   COMETS.  143 


• 
•'" 


fhc  orbit  of  Encke's  comet  is  wholly  within  the  orbit  of  Jupiter,  while  that  of 
Biel.Vs  extends  but  a  short  distance  beyond  it  The  aphelion  distance  of  Haliey's  comet 
Is  3,400  millions  of  miles,  or  550  millions 

of  miles  beyond  the  orbit  of  Neptune.  ORBITS  OF  SEVERAL  COMETS. 

But  these  are  all  comets  of  short  periods. 

305.  The    number     of 
comets   belonging    to,   or 
that  visit  the  solar  system, 
is  very  great.     Some  have 
estimated  them  at  several 
millions.     When  we  con- 
sider that  most  comets  are 
seen    only   through    tele- 
scopes— an   instrument  of 
comparatively     modern 
date — and    that,   notwith- 
standing  this,   some    440 
are  mentioned  in  ancient 

annals  and  chronicles,  as  having  been  seen  with  the 
naked  eye,  it  is  probable  that  the  above  opinion  is  by  no 
means  extravagant.  It  is  supposed  that  not  less  than  650 
have  been  seen  at  different  times  since  the  birth  of 
Christ.  The  paths  of  only  about  140  have  been  deter- 
mined. 

The  extreme  difficulty  of  ooserving  comets  wnose  nearest  point  is  beyond  the  orbit 
of  Mars,  is  supposed  to  account  for  the  comparatively  small  number  that  have  been  seen 
without  that  limit;  and  the  proximate  uniformity  of  the  distribution  of  their  orbits 
over  the  space  included  within  the  orbit  of  Mars,  seems  to  justify  the  conclusion,  that 
though  seldom  detected  beyond  his  path,  they  are  nevertheless  equally  distributed 
through  all  the  spaces  of  the  solar  heavens.  Eeasoning  upon  this  hypothesis,  Prolessoi 
Arago  concludes  that  there  are  probably  seven  millions  of  comets  that  belong  to  01 
visit  the  solar  system. 

306.  The  directions  of  comets  are  as  variable  as  their 
forms  or  magnitudes.     They  enter  the  solar  system  from 
all  points  of  the  heavens.     Some  seem  to  come  up  from 
the  immeasurable  depths  below  the  ecliptic,  and,  having 
doubled  u  heaven's  mighty  cape,"  again  plunge  down- 
ward with  their  fiery  trains,  and  are  lost  for  ages  in  the 
ethereal  void.     Others  appear  to  come  down  from  the 
zenith  of  the  universe,  and,  having  passed  their  peri- 

they  pass  ?  (What  samples  given  in  cut  ?  WThere  does  the  orbit  of  Encke's 
comet  lie  ?  Of  Biela's  ?  Of  'Haliey's  ?) 

305.  The  number  of  comets?     What  estimate?     Why  probably  correct? 
Row  many  supposed  to  have  been  seen  since  the  birth  of  Christ  ?     (Why  so 
few  seen  ?     How  supposed  to  be  distributed  ?     What  conclusion  of  Arago?) 

306.  Direction  of  comets  ?    (Remark  of  late  writer?) 


144  ASTEONOMY. 


hclion,  reascend  far  above  all  human  vision.  Others 
again  are  dashing  through  the  solar  system,  in  all  possible 
directions,  apparently  without  any  prescribed  path,  or  any 
guide  to  direct  them  in  their  eccentric  wanderings.  In- 
stead of  revolving  uniformly  from  east  to  west,  like  the 
planets,  their  motions  are  direct,  retrograde,  and  in  every 
conceivable  direction. 

It  is  remarked  by  a  late  writer,  that  the  average  inclinations  of  all  the  planes  in 
which  the  comets  now  on  record  have  been  found  to  move,  is  about  90°.  This  lie  re- 
gards as  a  wonderful  instance  of  the  goodness  of  Providence,  in  causing  their  motions 
to  be  performed  in  a  manner  least  likely  to  come  in  contact  with  the  earth  and  the  other 
planets. 

307.  Of  t\\Q  physical  nature  of  comets,  little  is  known. 
That  they  are,  in  general,  very  light  and  vapory  bodies, 
is  evident  from  the  fact  that  stars  have  sometimes  been 
seen  even  through  their  densest  portions,  and  are  gene- 
rally visible  through  their  tails,  and  from  the  little  attrac- 
tive influence  they  exert  upon  the  planets  in  causing 
perturbations.     While  Jupiter  and  Saturn  often  retard 
and  delay  comets  for  months  in  their  periodic  revolutions, 
cornets  have  not  power,  in  turn,  to  hasten  the  time  of  the 
planets  for  a  single  hour ;  showing  conclusively  that  the 
relative  masses  of  the  comets  and  planets  are  almost  in- 
finitely disproportionate. 

Such  is  the  extreme  lightness  or  tenuity  of  cometary  bodies,  that  in  all  probability 
the  entire  mass  of  the  largest  of  them,  if  condensed  to  a  solid  substance,  would  not 
amount  to  more  than  a  few  hundred  pounds.  Sir  Isaac  Newton  was  of  opinion,  that  if 
the  tail  of  the  largest  comet  was  compressed  within  the  space  of  a  cubic  inch,  it  would 
not  then  be  as  dense  as  atmospheric  air!  The  comet  of  1770  got  entangled,  by  attrac- 
tion, among  the  moons  of  Jupiter,  on  its  way  to  the  sun,  and  remained  near  them  for 
four  mtHUM ;  yet  it  did  not  sensibly  affect  Jupiter  or  his  moons.  In  this  way  the 
orMte  of  comets  are  often  entirely  changed. 

308.  Comets  were  formerly  regarded  as  harbingers  of 
famine,  pestilence,  war,  and  other  dire  calamities.     In 
one  or  two  instances,  they  have  excited  serious   appre- 
hension that  the  day  of  judgment  was  at  hand,  and  that 
they  were  the  appointed  messengers  of  Divine  wrath, 
hasting  apace  to  burn  up  the  world.     A  little  reflection, 
however,  will  show  that  all  such  fears  are  groundless. 
The  same  unerring  hand  that  guides  the  ponderous  planet 

807.  Physical  nature  of  comets  ?  What  proofs  of  their  light  and  vapory 
character?  (What  said  of  their  probable  mass?  Opinion  of  Newton? 
What  said  of  the  comet  of  1770  ?  What  effect  on  orbits  ?) 

308.  How  comets  formerly  regarded  ?  Why  no  fears  of  collision  ("Wh.it> 
estimate  of  'chances2"} 


OF   COMETS.  .115 


m  its  way,  directs  also  the  majestic  comet ,  and  where 
infinite  wisdom  and  almighty  power  direct,  it  is  almost 
profane  to  talk  of  collision  or  accident. 

Even  those  who  have  calculated  the  "  chances"  of  collision — as  if  chance  had  any 
thing  to  do  among  the  solar  bodies — have  concluded  the  chances  of  collision  are  about 
as  one  to  281,000,000— i.  e.,  like  the  chance  one  would  have  in  a  lottery,  where  there 
were  281,000,000  black  balls,  and  but  one  white  one ;  and  where  the  white  ball  must  be 
produced  at  the  first  drawing  to  secure  a  prize. 

309.  Were  a  collision  actually  to  take  place  between 
a  comet  and  the  earth,  it  is  not  probable  that  the  former 
would  even  penetrate  our  atmosphere,  much  less  dash 
the  world  to  pieces.     Prof.  Olinsted  is  of  opinion  that 
in  such  an  event,  not  a  particle  of  the  comet  would  reach 
the  earth — that  the  portions  encountered  by  her  would 
be  arrested  by  the  atmosphere,  and  probably  inflamed ; 
and  that  they  would  perhaps  exhibit,  on  a  more  magnifi- 
cent scale  than  was  ever  before  observed,  the  phenomena 
of  shooting  stars  or  meteoric  showers.     The  idea,  there- 
fore, that  comets  are  dangerous  visitants  to  our  system, 
has  more  support  from  superstition  than  from  reason  or 
science. 

The  air  is  to  us  what  the  waters  are  to  fish.  Some  fish  swim  around  in  the  deep, 
while  others,  like  lobsters  and  oysters,  keep  on  the  bottom.  So  birds  wing  the  air, 
while  men  and  beasts  are  the  "  lobsters"  that  crawl  around  on  the  bottom.  Now  there 
is  no  more  probability  that  a  comet  would  pass  through  the  atmosphere,  and  injure  us 
upon  the  earth,  than  there  is  that  a  handful  offoff  or  vapor  thrown  down  upon  the  sur 
face  of  the  ocean,  would  pass  through  and  kill  the  shell-fish  at  the  bottom. 

310.  After  all  that  is  supposed  to  be  known  respecting 
comets,  it  must  be  admitted  that  they  are  less  under- 
stood than  any  other  bodies  belonging  to  our  system. 
"  What  regions  these  bodies  visit,  when  they  pass  beyond 
the  limits  of  our  view ;  upon  what  errands  they  come, 
when  they  again  revisit  the  central  parts  of  our  system ; 
what  is  the  difference  between  their  physical  constitution 
and  that  of  the  sun  and  planets ;  and  what  important 
ends  they  are  destined  to  accomplish  in  the  economy  of 
the  universe,  are  inquiries  which  naturally  arise  in  the 
mind,  but  which  surpass  the  limited  powers  of  the  human 
understanding  at  present  to  determine." 

809.  What  probable  effect  in  case  of  collision?  Prof.  Olmsted's  opinion  ? 
(Remark  respecting  the  air,  fish,  lobsters,  &c.  ?) 

310.  Are  we  as  well  acquainted  with  comets  as  with  other  bodies  of  our 
eystem  2  What  inquiries  suggested  ?  How  answered  ? 

7 


110 


ASTRONOMY. 


CHAPTER    X. 


OF    THE     SUN. 

311.  OF  all  the  celestial  objects  with  which  we  an; 
acquainted,  none  make  so  strong  and  universal  an  im- 
pression upon  our  globe  as  does  the  sun.     He  is  the  great 
center  of  the  solar  system — a  vast  and  fiery  orb,  kindled 
by  the  Almighty  on  the  morn  of  creation,  to  cheer  tlu» 
dark  abyss,  and  to  pour  his  radiance  upon  surrounding 
worlds.     Compared  with  him,  all  the  solar  bodies  are  of 
inconsiderable  dimensions  ;  and  without  him,  they  would 
be  wrapped  in  the  gloom  of  interminable  night. 

312.  The  form  of  the  sun  is  that  of  an  oblate  sphe- 
roid, his  equatorial  being  somewhat  greater  than   his 
polar  diameter.     The  mean  of  the  two  is  886,000  miles. 
He  is  1,400,000  times  as  large  as  the  mighty  globe  we 
inhabit,  and  500  times"  as  large  as  all  the  planets  put 
together.     Were    he    placed 

where,  the  earth  is,  he  would 
fill  all  the  orbit  of  the  moon, 
and  extend  200,000  miles  be- 
yond it  in  every  direction.  It 
would  take  112  such  worlds 
as  ours,  if  laid  side  by  side,  to 
reach  across  his  vast  diameter. 

1.  The  vast  magnitude  of  the  sun  may  be 
inferred  from  the  fact,  that  when  rising  or  set- 
ting, he  often  appears  larger  than  the  largest 
building,  or  the  tops  of  the  largest  trees.    Now 
if  the  angle  filled  by  him  at  the  distance  of  two 
miles  is  over  100  feet  across,  what  must  it  be 
at  the  distance  of  95  millions  of  miles? 

2.  "Were  a  railroad  passed  through  the  sun's  center,  and  should  a  train  of  cars  start 
from  one  side,  and  proceed  on  at  the  rate  of  30  miles  an  hour,  it  would  require  34  years 

811.  Describe  the  sun.    How  compare  with  the  rest  of  the  system  ? 

312.  What  is  his  form?  Diameter?  Mass,  jus  compared  with  our  globe? 
With  all  other  bodies  of  the  system  ?  With  moon's  orbit  ?  (What  sensible 
evidence  of  *he  vast  magnitude  of  the.  sun?  Illustration  from  railroad8 
Pemor^tration  as  to  its  comparison  with  moon's  orbit  0 


THK  SUN  AND  THE  MOON'b  ORBIT. 


OF  THE   SUN. 


BPOTS   ON   THE    SUN. 


t.o  cross  over  ms  diameter.    To  traverse  his  vast  circumference,  at  the  same  rate  ol 
speed,  would  require  nearly  11  years. 

8.  The  mean  distance  of  the  moon  from  the  earth's  center  is  240.000  miles :  conse- 
quently the  diameter  of  her  orbit,  which  is  twice  the  radius,  is  480,000.  Subtract  this 
from  886,000,  the  sun's  diameter,  and  we  have  406,000  miles  left,  or  203,000  miles  on  each 
side,  beyond  the  moon's  orbit 

313.  By  the  aid  of  telescopes,  a  variety  of  spots  have 
been  discovered  upon  the  sun's  disk.     Their  number  is 
exceedingly  variable  at  different  times.     From  1611  to 
1629,  a  period  of  18  years,  the  sun  was  never  found  clear 
of  spots,  except  for  a  few 

days  in  December,  1624. 
At  other  times,  twenty  or 
thirty  were  frequently  seen 
at  once  ;  and  at  one  period, 
in  1825,  upwards  of  fifty 
were  to  be  seen.  Prof. 
Olmsted  states  that  over 
100  are  sometimes  visible. 
From  1650  to  1670,  a  pe- 
riod of  20  years,  scarcely 
any  spots  were  visible  ;  and 
for  eight  years,  from  1676 
to  1684,  no  spots  whatever 
were  to  be  seen.  For  the  last  46  years,  a  greater  or  less 
number  of  spots  have  been  visible  every  year.  For 
several  days,  during  the  latter  part  of  September,  1846, 
we  could  count  sixteen  of  these  spots,  which  were  dis- 
tinctly visible,  and  most  of  them  well  defined ;  but  on 
the  7th  of  October  following,  only  six  small  spots  were 
visible,  though  the  same  telescope  was  used,  and  circum- 
stances were  equally  favorable. 

The  sun  is  a  difficult  object  to  view  through  a  telescope,  even  when  the  eye  is  pro- 
tected in  the  best  manner  by  colored  glasses.  In  some  cases  (as  in  one  related  to  the 
author  by  Professor  Caswell,  of  Brown  University),  the  heat  becomes  so  great  as  to 
spoil  the  eye-pieces  of  the  instrument,  and  sometimes  the  eye  of  the  observer  is  irrepa- 
rably injured. 

314.  The  solar  spots  are  all  found  within  a  zone  60° 
wide — i.  e.,  30°  each  side  of  the  sun's  equator.    They  are 
generally  permanent,  though  they  have  been  known  to 

313.  View  of  sun's  surface  through  telescopes  ?    Number  of  spots  seen  ? 
Are  they  always  to  be  seen?    How  from  Ittll  to  1629?    In  1825?    Prof. 
OlmstedV  statement  ?     How  from  1050  to  1670?    From  1676  to  1684?    Ii. 
1846  ?     (What  said  of  difficulties  of  observing  ?) 

314.  Where  are  these  spots  situated  ?    Are  they  permanent  ?    What  mo- 


148  ASTRONOMY. 


un  revolves  in  the  direction  of  the  arrows,  and  in  25  days  10  hours  the  spot  comes 


break  in  pieces,  and  disappear  in  a  very  short  time. 
They  sometimes  break  out  again  in  the  same  places,  or 
where  none  were  perceptible  before.  They  pass  from 
left  to  right  over  the  sun's  disk  in  13  days,  15  hours, 
and  45  minutes  ;  from  which  it  has  been  ascertained  that 
he  performs  a  sidereal  revolution  on  his  axis,  from  west 
to  east,  or  in  the  di- 

'  -  -,  '         BIDEEKAL  AND  SYNODIC  INVOLUTIONS  OF  THE  BUN. 

rection    of    all     the 
planets,     every      25       E  / 
days,    7   hours,   and    *  ®-^ 

«/      '  a/Dm- 

48  minutes. 

1.  His  apparent  or  synodic 
revolution   requires    27  days  7£ 
hours;  but  this  is  as  much  more 
than  a  complete  revolution  upon 
his  axis,   as  the  earth   has    ad- 
vanced in  her  orbit  in  25  days  8 
hours.     Let  S  represent  the  sun, 
and  A  the   earth   in   her  orbit. 
When  she  is  at  A,  a  spot  is  seen 
upon  the  disk  of  the  sun  at  B. 
The  sun  revolves  in  the  direction 

round  to  B  again,  or  opposite  the  star  E.    This  is  a  ftidereal  revolution. 

2.  During  these  25  days  8  hours,  the  earth  has  passed  on  in  her  orbit  some  25°,  or 
m-arly  to  0,  which  will  require  nearly  two  days  for  the  spot  at  B  to  get  directly  toward 
the  earth,  as  shown  at  D.    This  last  is  a  synodic  revolution.    It  consists  of  one  com- 
plete revolution  of  the  sun  upon  his  axis,  and  about  27°  over. 

315.  Of  the  nature  of  these  wonderful  spots,  a  variety 
of  opinions  have  prevailed,  and  many  curious  theories 
have  been  constructed.  Lalande,  as  cited  by  Herschel, 
suggests  that  they  are  the  tops  of  mountains  on  the  sun's 
surface,  laid  bare  by  fluctuations  in  his  luminous  atmos- 
phere ;  'and  that  the  penumbrse  are  the  shoaling  declivi- 
ties of  the  mountains,  where  the  luminous  fluid  is  less 
deep.  Another  gentleman,  of  some  astronomical  knowl- 
edge, supposes  that  the  tops  of  the  solar  mountains  are 
exposed  by  tides  in  the  sun's  atmosphere,  produced  by 
planetary  attraction. 

To  the  theory  of  Lalande,  Dr.  Herschel  objects  that 
it  is  contradicted  by  the  sharp  termination  of  both  the  in- 
ternal and  external  edges  of  the  penumbraa ;  and  ad 
vances  as  a  more  probable  theory,  that  "  they  are  the 

tion  have  they?  What  conclusion  from  it?  (What  revolution  is  this? 
What  time  required  for  a  synodic  revolution  ?  Illustrate.) 

315.  What  are  these  spots  supposed  to  be  ?    Lalande  ?  &c.    Dr.  Herschel'a 
'e-oark?    Prof  Olmsted?    Prof.  Wilson?    Experiments  of  Prof.  Henry ? 


OF   THE   SUN.  149 


divrk,  or,  at  least,  comparatively  dark,  solid  body  of  the 
Bim  itself,  laid  bare  to  our  view  by  those  immense  fluc- 
tuations in  the  luminous  regions  of  the  atinospnere,  to 
which  it  appears  to  be  subject."  Prof.  Olmsted  supports 
this  theory  by  demonstrating  that  the  spots  must  bo 
"  nearly  or  quite  in  contact  with  the  body  of  the  sun." 

In  1773,  Prof.  Wilson,  of  the  University  of  Glasgow, 
ascertained,  by  a  series  of  observations,  that  the  spot 
were  probably  u  vast  excavations  in  the  luminous  matter 
of  the  sun  ;"  the  nuclei  being  their  bottom,  and  the  urn* 
brae  their  shelving  sides.  This  conclusion  varies  but 
little  from  that  of  Dr.  Herschel,  subsequently  arrived  at. 

In  a  series  of  experiments  conducted  by  Prof.  Henry, 
of  the  Smithsonian  Institute,  at  Washington,  by  means 
of  a  thermo-electrical  apparatus,  applied  to  an  image 
of  the  sun  thrown  on  a  screen  in  a  dark  room,  it  was 
found  that  the  spots  were  perceptibly  colder  than  the 
surrounding  light  surface. 

316.  The  magnitude  of  the  solar  spots  is  as    ariable 
as  their  number.     Upon  this  point,  the  second  cat  pre- 
ceding gives  a  correct  idea,  as  it  is  a  pretty  accurate  rep- 
resentation of  the  sun's  disk,  as  seen  by  the  writer  on  the 
22d  of  September,  1846.     In  1779,  Dr.  Herschel  ob- 
served a  spot  nearly  30,000  miles  in  breadth ;  and  he 
further  states,  that  others  have  been  observed,  whose 
diameter  was  upward  of  45,000  miles.     Dr.  Dick  ob- 
serves that  he  has  several  times  seen  spots  which  were 
not  less  than  ^  of  the  sun's  diameter,  or  22,192  miles 
across. 

It  is  stated,  upon  good  authority,  that  solar  spots  have 
been  seen  by  the  naked  eye — a  fact  from  which  Dr. 
Dick  concludes  that  such  spots  could  not  be  less  than 
50,000  miles  in  diameter.  The  observations  of  the 
writer,  as  above  referred  to,  and  represented  in  the  cut, 
would  go  to  confirm  this  deduction,  and  to  assign  a  still 
greater  magnitude  to  some  of  these  curious  and  interest- 
ing phenomena. 

317.  The  axis  of  the  sun  is  inclined  to  the  ecliptic  7J°, 

316.  What  said  of  the  size  of  the  solar  spots?  Dr.  Herschel'a  observa 
tions  ?  Dr.  Disk's  8  The  writer's  ? 


150  ASTRONOMY. 


or,  more  accurately,  7°  20'.  This  is  but  a  slight  deviation 
from  what  we  may  call  a  perpendicular  ;  so  that,  in  rela- 
tion to  the  earth,  he  may  be  considered  as  standing  up 
and  revolving  with  one  of  his  poles  resting  upon  a  point, 
just  half  his  diameter  below  the  ecliptic. 

As  the  result  of  the  sun's  motion  upon  his  axis,  his 
spots  always  appear  first  on  his  eastern  limb,  and  pass  off 
or  disappear  on  the  west.  But  though  the  direction  of 
the  spots,  as  viewed  from  the  earth,  is  from  east  to  west, 
it  only  proves  his  motion  to  coincide  with  that  of  the 
earth,  wrhich  we  call  from  west  to  east;  as  when  two 
spheres  revolve  in  the  same  direction,  the  sides  toward 
each  other  will  appear  to  move  in  opposite  directions. 
During  one-half  of  the  passage  of  the  spots  across  the 
sun's  disk,  their  apparent  motion  is  accelerated  /  and 
during  the  remainder,  it  is  retarded. 

This  apparent  irregularity  in  the  motion  of  the  spots 
upon  the  sun's  surface,  is  the  necessary  result  of  an 
equable  motion  upon  the  surface  of  a  globe  or  sphere. 
"When  near  the  eastern  limb,  the  spots  are  coming  partly 
toward  us,  and  their  angular  motion  is  but  slight ;  but 
wrhen  near  the  center,  their  angular  and  real  motions  are 
equal.  So,  also,  as  the  spots  pass  on  to  the  west,  it  is 
their  angular  motion  only  that  is  diminished,  while  the 
motion  of  the  sun  upon  his  axis  is  perfectly  uniform. 

318.  The  figure  of  the  sun  affects  not  only  the  appa- 
rent velocity  of  the  spots,  but  also  their  forms.     When 
first  seen  on  the  east,  they  appear  narrow  and  slender,  as 
represented  in  the  cut,  page  147.      As  they  advance 
westward,  they  continue  to  widen  or  enlarge  till  they 
reach  the  center,  where  they  appear  largest;  when  they 
again  begin  to  contract,  and  are  constantly  diminished, 
till  they  disappear. 

319.  Another  result  of  the  revolution  of  the  sun  upon 
an  axis  inclined  to  the  ecliptic,  and  the  revolution  of  the 

317.  How  is  the  sun's  axis  situated  ?    What  said  of  the  direction  of  the 
c<pot*  ?    Of  their  rate  of  motion  ? 

318.  Of  the  cause  of  this  irregularity !    What  variations  in  the  forms  <.  f 
the  solar  spots  ?    Cause  ? 

319.  What  other  result  of  the  sun's  revolution  about  an  inclined 
(Illustrate  by  diagrams.) 


OF   THE   SUN. 


151 


earth  around  him,  is,  that  when  viewed  from  our  mov- 
able observatory,  the  earth,  at  different  seasons  of  the 
year,  the  direction  of  the  spots  seems  materially  to  vary 


VAUIOUS  DIRECTIONS  OP  TUB  SOLAS  SPOTS. 


March. 


June 


September 


December. 


1.  Let  E  F  represent  the  plane  of  the  ecliptic.     In  March,  the  spots  describe  a  curve, 
which  is  convex  to  the  south,  as  shown  at  A.     In  June,  they  cross  the  sun's  disk  in 
nearly  straight  lines,  but  incline  upward.     In  September,  they  curve  again,  though  in 
the  opposite  direction;  and  in  December,  pass  over  in  straight  lines,  inclining  down- 
ward.   The  figures  B  and  D  show  the  inclination  of  the  sun's  axis. 

2.  The  cause  of  this  difference  in  the  direction  of  the  solar  spots  will  be  fully  under- 
stood by  the  following  diagram : 

SOLAR  SPOTS   OBSERVED  FROM  DIFFERENT  POINTS. 


Let  the  student  imagine  himself  stationed  upon  the  earth  at  A,  in  March,  looking 
upon  the  sun  in  the  center,  whose  north  or  upper  pole  is  now  inclined  tmcard  him. 
The  spots  will  then  curve  downward.  Three  months  afterward — viz.,  in  June — the 
earth  will  be  at  B;  when  the  sun's  axis  will  incline  to  t/ie  left,  and  the  spots  seem  to 
p:vss  upward  to  the  right  In  three  mouths  longer,  the  observer  will  be  at  C,  when  the 
north  pole  of  the  sun  willincline/;v?ra  him,  and  the  spots  seem  to  curve  upward;  and 
in  three  months  longer,  he  will  be  at  D,  when  the  axis  of  the  sun  will  incline  to  th* 
right,  and  the  spots  seem  to  incline  downward 

320.  Of  the  physical  constitution  of  the  sun,  very  lit- 
tle is  known.  When  seen  through  a  telescope,  it  is  like 
a  globe  of  fire,  in  a  state  of  violent  commotion  or  ebu- 
lition.  La  Place  believed  it  to  be  in  a  state  of  actual 
combustion,  the  spots  being  immense  caverns  or  craters, 
caused  by  eruptions  or  explosions  of  elastic  fluids  in  the 
interior. 

320.  "What  said  of  the  physical  constitution  of  the  sun?  La  Placo's  opin- 
ion ?  Most  probable  opinion  I 


1 52  ASTRONOMY. 


The  most  probable  opinion  is,  that  the  body  of  the  sun 
is  opake,  like  one  of  the  planets  ;  that  it  is  surrounded 
by  an  atmosphere  of  considerable  depth ;  and  that  the 
light  is  sent  off  from  a  luminous  stratum  of  clouds,  float- 
ing above  or  outside  the  atmosphere.  This  theory  accords 
best  with  his  density,  and  with  the  phenomena  of  the 
solar  spots. 

321.  Of  the  temperature  of  the  sun's  surface,  Dr.  Her- 
schel  thinks  that  it  must  exceed  that  produced  in  fur- 
naces, or  even  by  chemical  or  galvanic  processes.  By 
the  law  governing  the  diffusion  of  light,  he  shows  that 
a  body  at  the  sun's  surface  must  receive  300,000  times 
the  light  and  heat  of  our  globe  ;  and  adds  that  a  far  less 
quantity  of  solar  light  is  sufficient,  when  collected  in  the 
focus  of  a  burning-glass,  to  dissipate  gold  and  platina  into 
vapor.  The  same  writer  observes  that  the  most  vivid 
llames  disappear,  and  the  most  intensely  ignited  solids 
appear  only  as  black  spots  on  the  disk  of  the  sun,  when 
held  between  him  and  the  eye.  From  this  circumstance 
he  infers,  that  however  dark  the  body  of  the  sun  may 
appear,  when  seen  through  its  spots,  it  may,  neverthe 
less,  be  in  a  state  of  most  intense  ignition.  It  does  not, 
however,  follow,  of  necessity,  that  it  must  be  so.  The 
contrary  is,  at  least,  physically  possible.  A  perfectly 
reflective  canopy  would  effectually  defend  it  from  the 
radiation  of  the  luminous  regions  above  its  atmosphere, 
and  no  heat  would  be  conducted  downward  through  a 
gaseous  medium  increasing  rapidly  in  density.  The 
great  mystery,  however,  is  to  conceive  how  so  enormous 
a  conflagration  (if  such  it  be)  can  be  kept  up  from  age 
to  age.  Every  discovery  in  chemical  science  here  leaves 
us  completely  at  a  loss,  or  rather  seems  to  remove  further 
from  us  the  prospect  of  explanation.  If  conjecture 
might  be  hazarded,  we  should  look  rather  to  the  known 
possibility  of  an  indefinite  generation  of  heat  by  friction, 
or  to  its  excitement  by  the  electric  discharge,  than  to 
any  actual  combustion  of  ponderable  fluid,  whether  solid 
or  gaseous,  for  the  origin  of  the  solar  radiation. 

321.  Sun's  temperature  ?  Dr.  Herschel's  idea?  What  reasoning  agai.'ibt 
/lid  opinion  ?  What  mystery  \ 


THE   ZODIACAL   LIGHT. 


153 


ZODIACAL  LIGET. 


822.  The  Zodiacal  Light  is  a  faint  nebulous  light,  re- 
sembling the  tail  of  a  comet,  or  the  milky  way,  which 
seems  to  be  reflected  from 
the  regions  about  the  sun, 
and  is  distinguishable  from 
ordinary  twilight.  Its  form 
is  that  of  a  pyramid  or 
cone,  with  its  base  toward 
the  sun,  and  inclined  slight- 
ly to  the  ecliptic.  It  seems 
to  surround  the  sun  on  all 
sides,  though  at  various 
depths,  as  it  may  be  seen 
in  the  morning  preceding 
the  sun,  as  well  as  in  the 
evening  following  him ; 
and  the  bases  of  the  cones, 
where  they  meet  at  the  sun,  are  much  larger  than  his 
diameter. 

323.  Of  the  nature  of  this  singular  phenomenon,  very 
little  is  positively  known.     It  was  formerly  thought  to 
be  the  atmosphere  of  the  sun.     Prof.  Nichol  says  :  "  O^ 
this,  at  least,  we  are  certain — the  zodiacal  light  is  a  phe- 
nomenon precisely  similar  in  kind  to  the  nebulous  atmos- 
pheres of  the  distant  stars,  &c."     Sir  John  Herschel  re- 
marks that  it  is  manifestly  of  the  nature  of  a  thin  len- 
ticularly-formed  atmosphere,  surrounding  the  sun,  and 
extending  at  least  beyond  the  orbit  of  Mercury,  and 
even  of  V enus.     He  gives  the  apparent  angular  distance 
of  its  vertex  from  the  sun,  at  from  40°  to  90° ;  and  the 
breadth  of  its  base,  from  8°  to  30°.     It  sometimes  ex- 
tends 50°  westward,  and  70°  east  of  the  sun  at  the  same 
time. 

324.  The  form  of  this  substance  surrounding  the  sun, 
and  which  is  sufficiently  dense  to  reflect  his  light  to  the 

322.  What  is  tlie  zodiacal  light  ?    Its  form  ?    When  seen  ? 

323.  Nature  of  this  light  ?    Former  opinion  ?  Prof.  Niehol's  remark?    BJ. 
Herschel's  ?    Its  extent  from  the  sun  ? 

324.  Farm  of  this  light  ?    How  situated  with  respect  to  sun's  axis,  &c.  ? 
(Illustrate  by  diagram.) 


154:  ASTRONOMY. 


LIGHT. 


earth,  seems  to  be  that  of  a  lens  /  or  rather  that  of  a 
huge  wheel,  thickest  at  the  center,  and  thinned  down 
to  an  edge  at  the  outer  extremities.  Its  being  seen 
edgewise,  and  only  one-half  FOBM,  BXTKNT>  Em,  ov  THB 
at  a  time,  gives  it  the  ap- 
pearance of  two  pyramids 
with  their  bases  joined  at 
the  sun.  It  is  an  interest- 
ing fact,  stated  by  Prof. 
Nichol,  that  this  light  or 
nebulous  body  lies  in  the 
plane  of  the  sun's  equator. 
A  line  drawn  through  its 
transverse  diameter,  or 
from  one  apex  of  the  pyra- 
mids to  the  other,  would 
cross  the  axis  of  the  sun 
at  right  angles.  This  fact 
would  seem  to  indicate  a  revolution  of  this  curious  sub- 
stance with  the  sun  upon  his  axis. 

Let  A,  in  the  above  cut,  represent  the  sun,  B  B  his  axis ;  then  C  C  will  represent  the 
extent,  and  D  D  the  thickness  of  this  curious  appendage. 

325.  At  the  meeting  of  the  American  Association  for 
the  Advancement  of  Science,  held  in  Providence,  R. 
I.,  August  18,  1855.     The  Rev.  George  Jones,  of  the 
U.  S.  navy,  read  an  elaborate  paper  upon  the  Zodiacal 
Light,   founded   upon  his  own  observations  during  a 
cruise  in  the  United  States'  steam  frigate  Mississippi, 
from  41°  K  lat.  to  52°  S.  lat.     From  a  record  of  331 
observations,  each  accompanied  by  a  drawing,  showing 
the  exact  form  and  position  of  the  Light  among  the 
stars,  Mr.  Jones  was  decided  in  the  conviction  that  the 
Zodiacal  Light  is  a  luminous  ring  around  the  earth,  like 
that  which  surrounds  the  planet  Saturn.     Prof.  Pierce, 
of  Harvard  College,  is  said  to  have  concurred  with  him 
in  this  opinion. 

326.  After  all  the  observations  that  have  been  made, 

325.  Mr.  Jones'  observations  ?     Extent  ?     Where  made,  and  when  ? 
Prof.  Pierce' s  reported  opinion  ? 


155 

and  the  theories  that  have  been  advanced,  it  must  be  ad- 
mitted that  the  subject  of  the  zodiacal  light  is  but  imper' 
iectly  understood.  Prof.  Olmsted  supposes  it  to  be  a 
nebulous  body,  or  a  thin  vapory  mass  revolving  around 
the  sun ;  and  that  the  meteoric  showers  which  have  oc- 
curred for  several  years  in  the  month  of  November,  may 
be  derived  from  this  body.  This  is  the  opinion  of  Arago, 
Biot,  and  others. 

The  best  time  for  observing  the  zodiacal  light  is  on 
clear  evenings,  in  the  months  of  March  and  April.  It 
may  be  seen,  however,  in  October,  November,  and  De- 
cember, before  sunrise  ;  and  also  in  the  evening  sky. 


327.  Although,  in  general  terms,  we  speak  of  the  sun 
as  the  fixed  center  of  the  system,  it  must  not  be  under- 
stood that  the  sun  is  absolutely  without  motion.     On  the 
contrary,  he  has  a  periodical  motion,  in  nearly  a  circular 
direction,  around  the  common  center  of  all  the  planetary 
bodies;  never  deviating  from  his  position  by  more  than 
twice  his  diameter.     1  rom  the  known  laws  of  gravita- 
tion, it  is  certain  that  the  sun  is  affected  in  some  measure 
by  the  attraction  of  the  planets,  especially  when  many 
of  them  are  found  on  the  same  side  of  the  ecliptic  at  the 
same  time ;  but  this  would  by  no  means  account  for  so 
great  a  periodical  motion. 

328.  In  addition  to  the  motion  above  described,  the 
sun  is  found  to  be  moving,  with  all  his  retinue  of  planets 
and  comets,  in  a  vast  orbit,  around  some  distant  and 
hitherto  unknown  center.      This  opinion  was  first  ad 
vanced,  we  think,  by  Sir  William  Herschel ;  but  the 
honor  of  actually  determining  this  interesting  fact  be- 
longs to  Struve,  who  ascertained  not  only  the  direction 
of  the  sun  and  solar  system,  but   also   their  velocity. 

326.  Is  this  subject  well  understood  as  yet?     Prof.  Olmsted's  theory? 
When  the  best  time  for  observing  the  zodiacal  light  ? 

327.  Is  the  sun  really  stationary  ?    What  motion  \    How  affected  by  plan- 
et^ 

828.  What  other  motion  ?    Who  first  advanced  the  opinion  that  he  had 
3-iyh  a  motion  ?    Who  demonstrated  it?    Toward  what  point  is  the  sun  and 


156  ASTRONOMY. 


The  point  of  tendency  is  toward  the  constellation  Her- 
cules, right  ascension  259°,  declination  35°.  The  ve- 
locity of  the  sun  in  space  is  estimated  at  8  miles  per 
second,  or  28,000  miles  per  hour.  Its  period  is  about 
18,200,000  years ;  and  the  arc  of  its  orbit,  over  which 
the  sun  has  traveled  since  the  creation  of  the  world, 
amounts  to  only  about  ^oVotn  Part  °f  m's  orbit,  or  about 
7  minutes — an  arc  so  small,  compared  with  the  whole,  as 
to  be  hardly  distinguishable  from  a  straight  line. 

329.  With  this  wonderful  fact  in  view,  we  may  no 
longer  consider  the  sun  as  fixed  and  stationaiy,  but  rather 
as  a  vast  and  luminous  planet,  sustaining  the  same  rela- 
tion to  some  central  orb  that  the  primary  planets  sustain 
to  him,  or  that  the  secondaries  sustain  to  tLeir  primaries. 
Nor  is  it  necessary  that  the  stupendous  mechanism  of 
nature  should  be  restricted  even  to  these  sublime  propor- 
tions. The  sun's  central  body  may  also  have  its  orbit, 
and  its  center  of  attraction  and  motion,  and  so  on,  till, 
as  Dr.  Dick  observes,  we  come  to  the  great  center  of  ail 
— to  the  THRONE  OF  GOD  ! 

Professor  Madler,  of  Dorpat,  in  Kussia,  has  recently  announced  as  a  discovery  that 
the  star  Alcyone,  one  of  the  seven  stars,  is  the  center  around  which  the  sun  and  sour 
Bystem  are  revolving. 


CHAPTER    XI. 

MISCELLANEOUS   REMARKS   UPON   THE    SOLAR    SYSTEM. 
NEBFLAR  THEORY  OF  THE  ORIGIN  OF  THE  SOLAR  SYSTEM. 

330.  IT  was  the  opinion  of  La  Place,  a  celebrated 
French  astronomer,  that  the  entire  matter  of  the  solar 
system,  which  is  now  mostly  found  in  a  consolidated 

solar  system  tending  ?    Its  velocity  ?    Period  of  revolution  ?    Amount  of  its 
progress  since  the  creation  of  the  world  ? 

829.  How,  thm,  should  the  sun  be  considered  ?    How  extend  the  analogy  ? 
What  further  recont  discovery,  and  by  whom  ? 

830.  State  the  "  nebular  theory"  of  the  origin  of  the  solar  system  ?    Who 
^rst  started  this  theory  ? 


ORIGIN  OF  THE  SOLAR  SYSTEM NEBULAR  THEORY.  157 


state,  in  the  sun  and  planets,  was  once  a  vast  nebula  or 
gaseous  vapor,  extending  beyond  the  orbits  of  the  most 
distant  planets — that  in  the  process  of  gradual  conden- 
sation, by  attraction,  a  rotary  motion  was  engendered 
and  imparted  to  the  whole  mass — that  this  motion  caused 
the  consolidating  matter  to  assume  the  form  of  various 
concentric  rings,  like  those  of  Saturn ;  and,  finally,  that 
these  rings  collapsing,  at  their  respective  distances,  and 
still  retaining  their  motion,  were  gathered  up  into  plan- 
ets, as  they  are  now  found  to  exist.  This  opinion  is  sup- 
posed to  be  favored,  not  only  by  the  fact  of  Saturn's 
revolving  rings,  but  by  the  existence  of  the  zodiacal  light, 
or  a  resisting  medium  about  the  sun ;  and  also  by  the 
character  of  irresolvable  or  planetary  nebulae,  hereafter 
to  be  described. 

331.  To  this  theory,  however,  there  are  many  plau- 
sible, if  not  insurmountable,  objections. 

(a.)  It.  seems  to  be  directly  at  variance  with  the  Mosaic 
account  of  the  creation  of  the  sun,  moon,  and  stars. 
The  idea  that  the  sun  and  all  the  planets  were  made  up, 
so  to  speak,  out  of  the  same  general  mass,  not  only 
throws  the  creation  of  this  matter  back  indefinitely  into 
eternity,  but  it  substitutes  the  general  law  of  attraction 
for  the  more  direct  agency  of  the  Almighty.  The  crea- 
tion spoken  of  in  the  Bible  thus  becomes  not  the  origi- 
nating of  things  that  did  not  previously  exist,  but  the 
mere  organization  or  arrangement  of  matter  already 
existing. 

(5.)  The  supposed  consolidation  of  the  nebulous  mass, 
in  obedience  to  the  general  law  of  attraction,  does  not 
of  itself  account  for  the  rotary  motion  which  is  an  essen- 
tial part  of  the  theory.  Under  the  influence  of  mere  at- 
traction, the  particles  must  tend  directly  toward  the  cen- 
ter of  the  mass,  and  consequently  could  have  no  tendency 
to  produce  a  rotary  motion  during  the  process  of  conden- 
sation. 

(c.)  The  variation   of  the  planetary  orbits  from  the 

331.  What  said  of  it?  State  the  first  objection  named?  The  second? 
Third?  Fourth  ?  Fifth  ?  What  remark  added  by  the  author? 

14 


158  ASTRONOMY. 


plane  of  the  sun's  equator  contradicts  the  nebular  theory. 
If  the  several  primary  planets  were  successively  thrown 
off  from  the  general  mass,  of  which  the  sun  is  a  part, 
they  could  not  have  been  separated  from  the  parent  body 
till  they  were  near  the  plane  of  its  equator.  Now,  as 
the  sun  is  assumed  to  be  a  part  of  the  same  mass,  re- 
volving still,  the  theory  would  require  that  the  portions 
now  separated  from  him,  and  called  planets,  should  still 
revolve  in  the  plane  of  his  equator.  But  instead  of  this, 
it  is  found  that  some  of  them  vary  from  this  plane  to  the 
amount  of  nearly  42°. 

(d.)  This  theory  assumes  not  only  that  the  primary 
planets  were  thrown  off  from  the  parent  mass  by  its 
rapid  revolution,  but  that  the  primaries,  in  turn,  threw 
off  their  respective  satellites.  These,  then,  should  all 
revolve  in  the  plane  of  the  planetary  equators  respect- 
ively, and  in  the  direction  in  which  their  primaries  re- 
volve. But  their  orbits  not  only  depart  from  the  plane 
of  the  equators  of  their  primaries  (Jupiter's  satellites 
excepted),  but  the  moons  of  Uranus  actually  have  a 
retrograde  or  backward  revolution. 

(<?.)  If  the  sun  and  planets  are  composed  of  what  was 
originally  the  same  mass,  it  will  be  necessary  to  show 
why  they  differ  so  materially  in  their  physical  natures — 
why  the  sun  is  self-luminous,  and  the  planets  opake. 

But  we  have  not  room  to  discuss  the  subject  at  length 
in  this  treatise.  It  is  but  justice,  however,  to  say,  that 
men  eminent  for  learning  and  piety  have  advocated 
the  nebular  theory,  in  the  belief  that  it  is  perfectly  con- 
sistent with  the  Mosaic  account  of  creation.  But  the 
writer  is  frank  to  state,  that  while  he  acknowledges  the 
force  of  some  of  the  considerations  urged  in  its  sup- 
port, he  has  not  yet  seen  reason  for  adopting  this  theory 
of  the  origin  of  the  solar  system.  "  Through  faith  we 
understand  that  the  worlds  were  framed  by  the  word  ol 
God  [not  by  the  law  of  gravitation],  so  that  things  which 
are  seen  were  not  made  of  things  which  do  appear  [or  of 
pre-existing  matter]." — Ileb.  xi.  3. 

332.  Upon  the  supposition  that  the  sun  and  planets 
were  created  as  they  are,  by  the  direct  act  of  God,  an 


WERE  THE  ASTEEOIDS  ORIGINALLY  ONE  PLANET?      159 


inquiry  at  once  arises  as  to  the  probable  extent  of  the 
creation  recorded  by  Moses.  Does  it  include  the  whole 
universe  ?  or  is  it  to  be  understood  as  applicable  only  to 
the  solar  system  ?  Upon  this  point  our  only  light  is,  that 
"  in  the  beginning  God  created  the  heavens  and  the 
earth" — that  he  not  only  made  the  sun  and  moon,  but 
that  "  he  made  the  stars  also ;"  and  that  when  these  were 
spoken  into  being,  God  had  "finished"  his  work.  (See 
Genesis,  1st  chapter.)  "  Thus  the  heavens  and  the  earth 
were  finished,  and  all  the  host  of  them."  It  seems  most 
probable,  therefore,  that  the  Mosaic  creation  includes  the 
whole  material  universe — that  when  God  "  laid  the  foun- 
dations of  the  earth,"  and  the  "  heavens  were  the  work 
of  his  hands,"  he  "  made  the  worlds  also  ;"  that  is,  they 
were  then  all  "  framed  by  the  word  of  God." 

WERE  THE  ASTEROIDS  ORIGINALLY  ONE  PLANET  ? 

333.  Some  very  curious  speculations  have  been -enter- 
tained by  astronomers  in  regard  to  the  origin  of  the 
Asteroids.     As  in  the  case  of  the  recently  discovered 
planet,  Neptune,  the  existence  of  a  large  planet  between 
the  orbit  of  Mars  and  Jupiter  was  suspected  before  the 
asteroids  were  known.     This  suspicion  arose  mainly  from 
the  seeming  chasm  that  the  absence  of  such  a  body  would 
leave  in  the  otherwise  well-balanced  solar  system.     The 
prediction  that  such  a  body  would  be  discovered  in  the 
future  stimulated  the  search  of  astronomers,  till  at  length, 
instead  of  one  large  planet,  eighty-jive  small  ones  have, 
one  after  another,  been  discovered. 

334.  From  certain  peculiarities  of  the  asteroids,  it  has 
been  considered  highly  probable  that  they  are  the  frag- 
ments of  one  large  planet,  which  has  been  burst  asunder 
by  some  great  convulsion  or  collision.     The  grounds  of 
this  opinion  are  as  follows : 

332.  What  other  interesting  question  started  ?    What  light  apon  this  sub- 
ject?   What  most  probable? 

333.  What  curious  speculation  respecting  the  asteroids  ?    What  suspicions 
before  any  of  them  were  discovercu  ? 

334.  What  opinion  respecting  the  origin  of  the  asteroids?    State  the 
grounds  of  this  opinion  hi  order. 


160  ASTRONOMY. 


(a.)  The  asteroids  are  much  smaller  than  i*ny  of  the 
other  primary  planets. 

(£.)  They  are  all  at  nearly  the  same  distance  from  the 
sun. 

(c.)  Their  periodic  revolutions  are  accomplished  in 
neany  the  same  time.  The  difference  of  their  periodic 
times"  is  not  greater  than  might  result  from  the  supposed 
disruption,  as  the  parts  thrown  forward  would  have  their 
motion  accelerated,  while  the  other  parts  would  be  thrown 
back  or  retarded  f  thus  changing  the  periodic  times  of 
both. 

(d.)  The  great  departure  of  the  orbits  of  the  asteroids 
from  the  plane  of  the  ecliptic  is  supposed  to  favor  the 
hypothesis  of  their  having  been  originally  one  planet,  the 
assumption  being  that  the  explosion  separating  the  ori- 
ginal body  into  fragments  would  not  only  accelerate  some 
portions  and  retard  others,  but  would  also  throw  them  out 
of  the  plane  of  the  original  orbit,  and  in  some  cases  still 
further  from  the  ecliptic. 

(e.)  Their  orbits  are  more  eccentric  than  those  of  the 
other  primaries.  Although  the  tables  show  the  eccen- 
tricity of  Uranus's  orbit  as  greater  in  miles  than  that  oi 
even' Juno  or  Pallas,  yet  when  we  consider  the  difference 
in  the  magnitude  of  their  orbits,  it  will  easily  be  seen 
that  his  orbit  is  less  elliptical  than  theirs. 

(f.)  The  orbits  of  Ceres  and  Pallas,  at  least,  cross  each 
other.  This,  if  we  except,  perhaps,  the  orbits  of  some 
of  the  comets,  is  a  perfect  anomaly  in  the  solar  system. 

335.  From  all  these  circumstances,  it  has  been  con- 
cluded that  the  asteroids  are  only  the  fragments  of  an 
exploded  world,  which  have  assumed  their  present  forms 
since  the  disruption,  in  obedience  to  the  general  laws  oi 
gravitation.  This  theory,  first  advanced  by  Dr.  Olbers,  is 
favored  by  Prof.  Nichol,  Dr.  Brewster,  Dr.  Dick,  and 
others ;  while  Sir  John  Herschel  observes  that  it  may 
serve  as  a  specimen  of  the  dreams  in  which  astronomers, 
like  other  speculators,  occasionally  and  harmlessly  in- 

335.  Who  was  the  author  of  this  theory  ?  What  distinguished  astrono- 
mers favor  it  ?  What  says  Sir  John  Herschel  ?  Eeinurk  of  I)r.  Dick  ?  Opin- 
ion und  remarks  of  the  author? 


ARE    THE     PLANETS     INHABITED?  161 


dulge.  Dr.  Dick  remarks  that  the  breaking  up  of  the 
exterior  crust  of  the  earth,  at  the  time  of  the  general 
deluge,  was  a  catastrophe  as  tremendous  and  astonishing 
as  the  bursting  asunder  of  a  large  planet.  In  view,  how- 
ever, of  the  harmony  and  order  that  everywhere  reign 
throughout  the  planetary  regions,  directing  the  pathway 
and  controlling  the  destiny  of  every  world,  it  is  hard  to 
believe  either  that  one  world  has  been  so  constructed 
as  to  explode,  like  a  vast  bomb-shell,  and  scatter  its  frag- 
ments over  the  regions  of  its  former  pathway ;  or  that 
He  who  guides  even  the  erratic  comet  has  allowed  a  pon- 
derous world  to  get  so  off  its  track,  as  to  dash  itself  to 
pieces  against  its  fellow  worlds. 

AEE  THE  PLANETS  INHABITED  BY  RATIONAL  BEINGS? 

336.  Upon  this  interesting  question,  it  must  be  ad- 
mitted that  we  have  no  positive  testimony.  /  The  argu- 
ment for  the  inhabitedness  of  the  planets  rests  wholly 
upon  analogy,  and  the  conclusion  is  to  be  regarded  only 
in  the  light  of  a  legitimate  inference.     Still,  it  fs  remark- 
able tuat  those  who  are  best  acquainted  with  the  facts  of 
astronomy  are  most  confident  that  other  worlds  as  well 
as  ours  are  the  abodes  of  intellectual  life.     Indeed,  as 
Dr.  Dick  well  remarks,  it  requires  a  minute  knowledge 
of  the  whole  scenery  and  circumstances  connected  with 
the  planetary  system,  before  this  truth  comes  home  to  the 
understanding  with  full  conviction. 

337.  The  analogies  from  which  it  is  concluded  that  all 
the  primary  planets,  at  least,  are  inhabited  by  rational 
beings,  are  the  following  : 

(a.)  The  planets  are  all  solid  todies  resembling  the 
earth,  and  not  mere  clouds  or  vapors. 

(5.)  They  all  have  &  spherical  or  spheroidal  figure,  like 
our  own  planet. 

(<?.)  The  laws  of  gravitation,  by  which  we  are  kept 
upon  the  surface  of  the  earth,  prevail  upon  all  the  other 

836.  What  other  Question  proposed?    What  admission?    Nature  of  the 
evidence  of  the  inhabitedness  of  the  planets  ?    What  remarkable  fact?    Ko 
mark  of  Dr.  Dick  ? 

837.  State  the  principal  points  of  analogy  between  our  globe  and  the  othe; 


162  ASTRONOMY. 


planets,  as  if  to  bind  races  of  material  beings  to  their  sur- 
faces, and  provide  for  the  erection  of  habitations  and 
other  conveniences  of  life.  It  is  very  remarkable,  how- 
ever, that  those  planets  whose  bulks  are  such  as  to  indi- 
cate an  insupportable  attractive  force,  are  not  only  less 
dense  than  our  globe,  but  they  have  the  most  rapid  daily 
revolution  ;  as  if,  by  diminished  density,  and  a  strong 
centrifugal  force  combined,  to  reduce  the  attractive  force, 
and  render  locomotion  possible  upon  their  surfaces.  . 

(d.)  The  magnitudes  of  the  planets  are  such  as  to  af- 
ford ample  scope  for  the  abodes  of  myriads  of  inhabit- 
ants. It  is  estimated  that  the  solar  bodies,  exclusive  of 
the  comets,  contain  an  area  of  78,000,000,000  of  square 
miles,  or  397  times  the  surface  of  our  globe.  According 
to  the  population  of  England,  this  vast  area  would  afford 
a  residence  to  21,875,000,000,000  of  inhabitants;  or 
27,000  times  the  population  of  our  globe. 

(e.)  The  planets  have  a  diurnal  revolution  around  < 
their  axes,  thus  affording  the  agreeable  vicissitudes  of 
day  and  night  Not  only  are  they  opake  bodies  like  our 
globe,  receiving  their  light  and  heat  from  the  sun,  but 
they  also  revolve  so  as  to  distribute  the  light  and  shade 
alternately  over  each  hemisphere.  There,  too,  the  glo- 
rious sun  rises  to  enlighten,  warm,  and  cheer  ;  and  there 
"  the  sun-strown  firmament"  of  the  more  distant  heavens 
is  rendered  visible  by  the  no  less  important  blessing  of  a 
periodic  night. 

(/".)  All  the  planets  have  an  annual  revolution  round 
the  sun ;  which,  in  connection  with  the  inclination  of 
their  axes  to  their  respective  orbits,  necessarily  results  in 
the  production  of  seasons. 

(g.)  The  planets,  in  all  probability,  are  enveloped  in 
atmospheres.  That  this  is  the  case  with  many  of  them 
is  certain ;  and  the  fact  that  a  fixed  star,  or  any  other 
orb,  is  not  rendered  dim  or  distorted  wrhen  it  approaches 
their  margin,  is  no  evidence  that  the  planets  have  no  at- 
mosphere. Tiiis  appendage  to  the  planets  is  known  to 
vary  in  density;  and  in  those  cases  where  it  is  no";  de- 

[>lanets.  Substance  ?  Forms  ?  Gravitation  ?  Magnitude  ?  Day*  and 
aights?  Seasons?  Atmospheres?  Moons?  Mountains?  &e. 


ARE     THE     PLANETS     INHABITED?  163 


tected  by  its  intercepting  or  refracting  the  light,  it  may 
be  of  a  nature  too  clear  and  rare  to  produce  such  phe- 
nomena. 

(h.)  The  principal  primary  planets  are  provided  with 
moojis  or  satellites,  to  afford  them  light  in  the  absence  of 
the  sun.  It  is  not  improbable  that  both  Mars  and  Yenus 
have  each,  at  least,  one  moon.  The  earth  has  one  ;  and 
as  the  distances  of  the  planets  are  increased,  the  number 
of  moons  seems  to  increase.  The  discovery  of  six  around 
Uranus,  and  only  one  around  Neptune,  is  no  evidence 
that  others  do  not  exist  which  have  not  yet  been  dis- 
covered. 

(i.)  The  surfaces  of  all  the  planets,  primaries  as  well 
as  secondaries,  seem  to  be  variegated  with  hill  and  dale, 
mountain  and  plain.  These  are  the  spots  revealed  by 
the  telescope. 

(j.)  Every  part  of  the  globe  we  inhabit  is  adapted 
to  the  support  of  animal  life.  It  would,  therefore,  be 
contrary  to  the  analogy  of  nature,  as  displayed  to  us,  to 
suppose  that  the  other  planets  are  empty  and  barren 
wastes,  utterly  devoid  of  animated  being.  And  if  ani- 
mals of  any  kind  exist  there,  why  not  intelligent  beings  2 

338.  If  other  worlds  are  not  the  abodes  of  intellectual 
life,  for  what  were  they  created  ?  What  influence  do 
they  exert  upon  our  globe,  especially  those  most  remote  ? 
There  are  doubtless  myriads  of  worlds  beyond  our  system 
that  will  never  even  be  seen  by  mortal  eye,  and  that  have 
no  perceptible  connection  with  our  globe.  If,  then,  they 
are  barren  and  uninhabited  islands  in  the  great  ocean  of 
immensity,  we  repeat,  for  what  were  they  created  ?  The 
inquiry  presses  itself  upon  the  mind  with  irresistible 
force,  Why  should  this  one  small  world  be  inhabited, 
and  all  the  rest  unoccupied  ?  For  what  purpose  were  all 
these  splendid  and  magnificent  worlds  fitted  up,  if  not  to 
be  inhabited  ?  Why  these  days  and  years — this  light  and 
shade — these  atmospheres,  and  seasons,  and  satellites,  and 
hill  and  dale? 

338.  What  difficulty  on  the  supposition  that  the  planets  are  not  inha'  - 
ited  1 


164:  ASTRONOMY. 


339.  To  suppose  all  these  worlds  to  be  fitted  up  upon 
one  general  plan,  provided  with  similar  conveniences  as 
abodes  for  intellectual  beings,  and  yet  only  one  of  them 
to  be  inhabited,  is  like  supposing  a  rich  capitalist  would 
build  some  thirty  fine  dwellings,  all  after  one  model, 
though  of  different  materials,  sizes,  and  colors,  and  pro- 
vide in  all  for  light,  warmth,  air,  &c. ;  and  yet,  having 
placed  the  family  of  a  son  in  one  of  them,  allow  the 
remaining  twenty-nine  to  remain   unoccupied  forever ! 
And  as  God  is  wiser  than  man,  in  the  same  proportion 
does  it  appear  absurd,  that  of  nearly  ninety  planetary 
temples  now  known  to  exist,  only  one  has  ever  been  occu 
pied;  while  the  remainder  are  mere  specimens  of  Divine 
architecture,  wheeling  through  the  solitudes  of  immen- 
sity !     The  legitimate  and  almost  inevitable  conclusion, 
therefore,  is,  that  our  globe  is  only  one  of  the  many 
worlds  which  God  has  created  to  be  inhabited,  and  which 
are  now  the  abodes  of  his  intelligent  offspring.     It  seems 
irrational  to  suppose  that  we  of  earth  are  the  only  intel 
ligent  subjects  of  the  "  Great  King,"  whose  dominions 
border  upon  infinity.     It  is  much  more  in  keeping  with 
sound  reason,  and  with  all  the  analogies  of  our  globe, 
to  suppose  that 

"  Each  revolving  sphere,  a  seeming  point, 
Which  through  night's  curtain  sparkles  on  the  eye, 
Sustains,  like  this  our  earth,  its  busy  millions." 

340.  The  fact  that  we  neither  see,  nor  hear,  nor  hear 
from  the  inhabitants  of  other  worlds,  is  no  evidence  that 
such  inhabitants   do   not  exist.     It  would  have  been 

Eremature  in  Columbus  had  he  concluded,  when  he  saw 
md  in  the  distance,  that  it  was  uninhabited,  simply  be- 
cause he  could  not  hear  the  shout  of  its  savages,  or  see 
them  gathered  in  groups  upon  the  beach.  So  in  regard 
to  the  distant  planets.  Our  circumstances  forbid  our 
knowing  positively  that  they  are  inhabited ;  so  that  the 
absence  of  that  knowledge  is  no  argument  against  the 
inhabitedness  of  other  worlds. 

339.  What  illustration  ?    Conclusion  ?    Poetry  ? 

840.  What  said  of  the  objection  that  we  neither  see,  hear,  nor  hear  from 
the  inhabitants  of  the  other  worlds  ? 


AKE    THE     PLANETS     INHABITED?  165 


341.  It  may  be  thought  that  the  extremes  of  heat  and 
cold  on  some  of  the  planets  must  be  fatal  to  the  idea  of 
animal  life,  at  least.  But  even  this  does  not  follow. 
Upon  our  globe,  some  animals  live  and  flourish  where 
others  would  soon  die  from  heat  or  cold.  And  some  ani- 
mals, having  cold  blood,  may  be  frozen,  and  yet  live. 
So  in  other  worlds.  He  who  made  the  three  Hebrews 
to  live  in  the  fiery  furnace,  can  easily  adapt  the  inhabit- 
ants of  Mercury  to  their  warm  abode.  And  of  the  exte 
rior  planets  we  have  only  to  say : 

"  Who  there  inhabit  must  have  other  powers, 
Juices,  and  veins,  and  sense,  and  life,  than  ours  ; 
One  moment's  cold,  like  theirs,  would  pierce  the  bone, 
Freeze  the  heart's  blood,  and  turn  us  all  to  stone  1" 

Adaptation  is  a  law  of  the  universe;  and  this  at  once 
obviates  every  difficulty  in  regard  to  the  temperature  of 
the  planets,  which  might  otherwise  be  urged  as  a  reason 
why  they  were  not  inhabited. 

841.  Objection  drawn  from  extremes  of  temperature?     Poetry?    What 
grev.  law  answers  every  such  objection? 


PART  II. 

THE  SIDEREAL  HEAVENS. 


CHAPTER  I. 

THE    FIXED    STARS CLASSIFICATION,  NUMBER,  DISTANCE,  ETC. 

34:2.  THE  sidereal  heavens  embrace  all  those  celestial 
bodies  that  lie  around  and  beyond  the  solar  system,  in 
the  region  of  the  fixed  stars. 

The  fixed  stars  are  distinguished  from  the  planetary 
bodies  by  the  following  characteristics  : 

(a.)  They  shine  by  their  own  light,  like  the  sun,  and 
not  by  reflection. 

(&.)  To  the  naked  eye,  they  seem  to  twinkle  or  scintil- 
late •  while  the  planets  appear  tranquil  and  serene. 

(c.)  They  maintain  the  same  general  positions,  with 
respect  to  each  other,  from  age  to  age.  On  this  account, 
they  are  called  fixed  stars. 

(d.)  They  are  inconceivably  distant/   so  that,  whel 
viewed  through  a  telescope,  they  present  no  sensible  disk, 
but  appear  only  as  shining  points  on  the  dark  concave  of 
the  sky.     To  these  might  be  added  several  other  peculi- 
arities, which  will  be  noticed  hereafter. 

343.  For  purposes  of  convenience,  in  finding  or  refer- 
ring to  particular  stars,  recourse  is  had  to  a  variety  ol 
artificial  methods  of  classification. 

842.  What  parts  of  the  book  have  we  now  gone  over  ?    Upon  what  do  we 
now  enter?     What  is  meant  by  the  sidereal  heaveus  ?    How  are  the  fixed 
stars  distinguished  from  planetary  bodies  ? 

843.  What  are  constellations  ?    Their  origin  ? 


THE   FIXED   STAKS CLASSIFICATION.  107 


First,  The  whole  concave  of  the  heavens  is  divided 
into  sections  or  groups  of  stars  of  greater  or  less  extent. 
The  ancients  imagined  that  the  stars  were  thrown  toge- 
ther in  clusters,  resembling  different  objects,  and  they 
consequently  named  the  different  groups  after  the  objects 
which  they  supposed  them  to  resemble.  These  clusters, 
when  thus  marked  out  by  the  figure  of  some  animal, 
person,  or  thing,  and  named  accordingly,  were  called 
constellations. 

344.  Secondly,  The  stars  are  all  classed  according  to 
their  magnitudes.  There  are  usually  reckoned  twelve 
different  magnitudes,  of  which  the  first  six  only  are 
visible  to  the  naked  eye,  the  rest  being  telescopic  stars. 
These  magnitudes,  of  course,  relate  only  to  their  apparent 
brightness  ;  as  the  faintest  star  may  appear  dim  solely  on 
account  of  its  immeasurable  distance.  The  method  by 
which  stars  of  different  magnitudes  are  distinguished  in 
astronomical  charts  is  as  follows : 


8TAB8  OF  DIFFERENT  MAGNITUDES. 

3  4  5         6       7     8     9    10  11  12 


"It  must  be  observed,"  says  Dr.  Herschel,  "thatttits  classification  in  to  magnitudes  Is 
entirely  arbitrary.  Of  a  multitude  of  bright  objects,  differing,  probably,  intrinsically 
botli  in  size  and  in  splendor,  and  arranged  at  unequal  distances  from  us,  one  must  oi 
necessity  appear  the  brightest ;  the  one  next  below  it  brighter  still,  and  so  on." 

345.  The  next  step  is  to  classify  the  stars  of  each  con 
stellation  according  to  their  magnitude  in  relation  to  each 
other,  and  without  reference  to  other  constellations.  In 
this  classification,  the  Greek  alphabet  is  first  used.  For 
instance,  the  largest  star  in  Taurus  would  be  marked  (a) 
Alpha ;  the  next  largest  (/3)  Beta  ;  the  next  (7)  Gamma, 
&c.  "When  the  Greek  alphabet  is  exhausted,  the  Roman 
or  English  is  tuken  up ;  and  when  these  are  all  absorbed, 
recourse  is  finally  had  to  figures. 

As  Greek  letters  so  frequently  occur  in  catalogues  and  maps  of  the  stars,  and  on  the 
celestial  globes,  the  Greek  alphabet  is  here  inserted,  for  the  benefit  of  those  who  are  not 

344.  How  classified  by  magnitudes  ?    (Remark  of  Dr.  Herschel  ?) 

345.  Next  step  iri  classifying  ?    How  conducted?    Greek  letters  \    (Eepou 
Iho  ulphubet.) 


168  ASTRONOMY. 


ccqnainted  with  it;  but  as  the  capitals  are  seldom  used  for  designating  tho  stars,  the 
small  characters  only  are  given  : 

THE  GREEK  ALPHABET. 


a  Alpha  a 

0  Beta  b 

r  Gamma  g 

Delta  d 

e  Epsilou  e  short 

$  Zeta  z 

rj  Eta  e  long 

e  Theta  th 

1  Iota 

K  Kappa  k 

X  Lambda  1 

M  Mu  m 


Nu  n 

Xi  x 

Omicron  o  short 

Pi  p 

Rho  T 

Sigma  s 

Tau  t 

Upsilon  n 

Phi  ph 

Chi  ch 

Psi  ps 

Omega  o  long 


346.  To  aid  in  finding  particular  stars,  and  especially 
in  determining  their  numbers,  and  detecting  changes, 
should  any  occur,  astronomers  have  constructed  cata- 
logues of  the  stars,  one  of  which  is  nearly  2,000  years 
old.     Several  of  the  principal  stars  have  a  specific  name 
—  as  Sirius,  Aldebaran,  Megulus,  &c.  ;  and  clusters  of 
stars  in   a   constellation   sometimes  receive  a  specific 
name,  as  the  Pleiades  and  Hyades  in  Taurus. 

347.  The  stars  are  still  further  divided  into  double, 
triple,  and   quadruple   stars,  binary  systems,  variable 
stars,  periodic  stars,  nebulous  stars,  &c.,  all  of  which 
will  be  noticed  hereafter. 

NUMBER   OF   THE   FIXED   STABS. 

348.  The  actual  number  of  the  stars  is  known  only  to 
Him  who  "  telleth  the  number  of  the  stars,  and  calleth 
them  all  by  their  names."    The  powers  of  the  human 
mind  are  barely  sufficient  to  form  a  vague  estimate  of 
the  number  near  enough  to  be  seen  by  our  best  tele- 
scopes, and  here  our  inquiries  must  end. 

The  number  of  stars,  down  to  the  twelfth  magnitude, 
has  been  estimated  as  follows  : 


846.  What  farther  methods  for  finding  particular  stars  ? 

847.  How  are  the  stars  still  further  distinguished  ? 

348.  Number  of  the  stars?     Of  each  magnitude?     Number  visible  to 
naked  eye  ?     Additional  seen  through  telescopes  ?     Total  ?     Kemarks  of 
hcl  and  Olmstcd  ? 


NUMBER     OF     THE     FIXED     STABS. 


169 


Visible  to  the  naked  eye. 

Visible  only  through  telescopes. 

1st  ?iiao-nitude 

18 

Yth  magnitude      26,000 

2d          " 

52 

8th         "             170,000 

3d          " 

177 

9th         «          1,100,000 

4th        " 

376 

10th         "          7,000,000 

5th        " 

1,000 

llth         "        46,000,000 

6th        « 

4,000 

12th         "      300,000,000 

Total 

5,623 

Grand  total,  354,301,623 

NUMBER  OF  8TAK8  OF  EACH  MAGNITUDE. 


Of  these  stars,  Dr.  Herschel  remarks  that  from  15,000 
to  20,000  of  the  first  seven  magnitudes  are  already  regis- 
tered^ or  noted  down  in  catalogues ;  and  Prof.  Olmsted 
observes  that  Lalande  has  registered  the  positions  of  no 
less  than  50,000. 

349.  The  reason  why  there  are  so  many  more  of  the 
binall  stars  than  of  the  large  ones  is,  that  we  are  in  the 
midst  of  a  great  cluster,  with  but  few  stars  near  us,  the 
number  increasing  as  the 
circumference  of  our 
view  is  enlarged.  (See 
second  cut,  page  28,  and 
also  the  adjoining.) 

Let  the  ;entral  star  represent  the 
sun  (a  star  only  among  the  rest),  with 
the  solar  system  revolving  between 
him  »nd  the  first  circle.  The  18  stars 
in  space  1st  will  apperr  to  be  of  the 
first  magnitude,  on  account  of  their 
nearness,  and  they  are  thus  few  be- 
cause they  emDrace  but  a  small  part 
of  the  entire  cluster.  The  stars  of 
space  2d  will  app*wr  smaller,  being 
more  distant;  but  as  it  embraces  more 
space,  they  will  be  more  numerous. 
Thus  as  we  advance  from  one  circle 
to  another,  the  apparent  magnitude 
constantly  diminishes,  but  the  num- 
ber constantly  increases.  The  large 
white  circle  marks  the  limit  of  our 
natural  vision.  Even  this  cut  foils  to  present  fully  to  the  eye  the  cause  of  the  rapid  in- 
crease in  numbers,  for  we  can  only  show  the  surface  of  a  cut  section  of  our  firmament 
of  stars,  which  exhibits  the  increase  in  a  plane  only,  whereas  our  sun  seems  to  be  im- 
bedded in  the  midst  of  a  magnificent  cluster  (like  a  single  apple  in  the  midst  of  a  large 
tree  richly  laden  with  fruit),  the  stars  of  which  we  vievv  around  us  in  every  direction. 


349.  Why  so  many  more  of  small  stars  than  of  the  larsrer  ?  (Illustrate  by 
diagram.  Does  this  convey  a  complete  idea  of  the  position  of  the  sun,  with 
reference  to  the  fixed  stars?  Whj  not?  What  docs  his  position  more  nearly 

8 


ITO  ASTKONOMY. 


350.  If  we  suppose  that  each  of  these  suns  is  accom- 
panied only  by  as  many  planets  as  are  embraced  in  onr 
solar  system,  we  have  nine  thousand  millions  of  worlds 
in  our  firmament.     ]STo  human  mind  can  form  a  concep- 
tion of  this  number ;  but  even  these,  as  will  hereafter  be 
shown,  form  but  a  minute  and  comparatively  insignifi- 
cant portion  of  the  boundless  empire  which  the  Creator 
has  reared,  and  over  which  he  reigns.     "  Lo,  these  are 
parts  of  his  ways ;  but  how  little  a  portion  is  heard  of 
fern?  but  the  thunder  of  his  power  who  can  under- 
stand."    (Job  xxvi.  14.) 

DISTANCES   AND   MAGNITUDES    OF   THE   STAKS. 

351.  It  has  been  demonstrated  that  the  nearest  of  tne 
fixed  stars  cannot  be  less  than  20,000,000,000  (twenty 
billions)  of  miles  distant !     For  light  to  travel  over  this 
space,  at  the  rate  of  200,000  miles  per  second,  would  re- 
quire 100,000,000  seconds,  or  upwards  of  three  years. 

What,  then,  must  be  the  distances  of  the  telescopic 
stars,  of  the  10th  and  12th  magnitudes  ?  "  If  we  admit," 
says  Dr.  Herschel,  "  that  the  light  of  a  star  of  each  mag 
nitude  is  half  that  of  the  magnitude  next  above  it,  it  will 
follow  that  a  star  of  the  first  magnitude  will  require  to 
be  removed  to  362  times  its  distance,  to  appear  no  larger 
than  one  of  the  twelfth  magnitude.  It  follows,  therefore, 
that  among  the  countless  multitude  of  such  stars,  visible 
in  telescopes,  there  must  be  many  whose  light  has  taken 
at  least  a  thousand  years  to  reach  us  ;  and  that  when  we 
observe  their  places,  and  note  their  changes,  we  are,  in 
fact,  reading  only  their  history  of  a  thousand  years' 
date,  thus  wonderfully  recorded."  Should  such  a  star  be 
struck  out  of  existence  now,  its  light  would  continue  to 
stream  upon  us  for  a  thousand  years  to  come  ;  and  should 
a  new  star  be  created  in  those  distant  regions,  a  thousand 
years  must  pass  away  before  its  light  could  reach  the 
solar  system,  to  apprise  us  of  its  existence. 


350.  What  supposition  and  conclusion  ?    Scripture  qnotation  ? 
S51.  Distances  of  the  nearest  stars?    Time  for  light  to  travel  over  tiu« 
space  ?    Suppositions  and  conclusions  of  Dr.  Ilersch^i  3 


MAGNITUDE   OF   THE   STARS.  171 


352.  From  what  we  have  already  said  respecting  the 
almost  inconceivable  distances  of  the  fixed  stars,  it  will 
readily  be  inferred  that  they  must  be  bodies  of  great 
magnitude,  in  order  to  be  visible  to  us  upon  the  earth. 
It  is  probable,  however,  that  "  one  star  differeth  from 
another"  in  its  intrinsic  splendor  or  "  glory,"  although 
we  are  not  to  infer  that  a  star  is  comparatively  small  be 
cause  it  appears  small  to  us. 

353.  The  prevailing  opinion  among  astronomers  is. 
that  what  we  call  the  fixed  stars  are  so  many  suns  and' 
centers  of  other  systems.     From  a  series  of  experiments 
upon  the  light  received  by  us  from  Sirius,  the  nearest  oi 
the  fixed  stars,  it  is  concluded  that  if  the  sun  were  re- 
moved 141,400  times  his  present  distance  from  us,  or 
to  a  point  thirteen  billions  of  miles  distant,  his  light 
would  be  no  stronger  than  that  of  Sirius  ;  and  as  Sirius 
is  more  than  twenty  billions  of  miles  distant,  he  must, 
in  intrinsic  magnitude  and  splendor,  be  equal  to  two  suns 
like  ours.     Dr.  Wollaston,  as  cited  by  Dr.  Herschel,  con- 
cludes that  this  star  must  be  equal  in  intrinsic  light  to 
nearly  fourteen  suns.     According  to  the  measurements 
of  Sir  Wm.  Herschel,  the  diameter  of  the  star  Vega  in 
the  Lyre  is  38  times  that  of  the  sun,  and  its  solid  con- 
tents 54,872  times  greater!     The  star  numbered  61  in 
the  Swan  is  estimated  to  be  200,000,000  miles  in  di- 
ameter. 

354.  Sir  John  Herschel  states,  that  while  making  ob- 
servations with  his  forty-feet  reflector,  a  star  of  the  first 
magnitude  was  unintentionally  brought  into  the  field  of 
view.     "  Sirius,"  says  he,  "  announced  his  approach  like 
the  dawn  of  day  ;"  and  so  great  was  his  splendor  when 
thus  viewed,  and  so  strong  was  his  light,  that  the  great 
astronomer  was  actually  driven  from  the  eye-piece  of  his 
telescope  by  it,  as  if  the  sun  himself  had  suddenly  burst 
upon  his  view. 

352.  Whal  inference  from  the  great  distance  of  the  stars  ?    What  proba- 
bility as  to  the  real  magnitude  c.f  the  stars  ? 

353.  The  prevailing  opinion  among  astronomers  ?    Conclusions  from  ex- 
periments with  Sirius?    Magnitude  of  Vega?    Of  No.  61  in  the  Swan? 

354.  Incident  stated  by  Dr.  ilerscliel?    (Relative  light  of  tne  stars  ol  the 
first  six  mugnitadea  ?) 


172  ASTRONOMY. 


According  to  Sir  Wm.  Herschel,  the  relative  light  of  the  stars  of  tho  first  sis  magni- 
tudes is  as  follows: 

Light  of  a  star  of  the  average  1st  magnitude 100 

8  *  «         2d  "        25 

«  «  "         3d  "        12 

4th          "        6 

«  "  "        5th          "        2 

«  "         6th          "        ....  .1 


CHAPTER     II. 


DESCRIPTION     OF     THE     CONSTELLATIONS. 

355.  ALTHOUGH  this  work  is  designed  particularly  to 
illustrate  the  mechanism  of  the  heavens,  as  displayed  in 
the   solar  system,  we   are  desirous   of  furnishing  the 
learner  with  a  sufficient  guide  to  enable  him  to  extend 
his  inquiries  and  investigations  not  only  to  the  different 
classes  of  bodies  lying  beyond  the  limits  of  the  solar 
system  in  the  far  off  heavens,  but  also  to  the  constella- 
tions, as  such.     For  this  purpose,  we  shall  here  furnish 
a  brief  description  of  the  principal  constellations  visible 
in  the  United  States,  or  in  north  latitude ;  by  the  aid  of 
which,  the  student  will  be  able  to  trace  them,  with  very 
little  difficulty,  upon  that  glorious  celestial  atlas  which 
the  Almighty  has  spread  out  before  us. 

If  the  student  will  be  at  the  trouble  to  identify  the  constellations  by  the  aid  of  these 
descriptions,  and  without  the  aid  of  chart?,  it  will  give  him  a  practical  familiarity  with 
the  heavens  which  can  be  acquired  in  no  other  way.  Indeed,  this  exercise  is  indispen- 
sable to  a  competent  knowledge  of  sidereal  astronomy,  even  where  maps  of  the  constel- 
lations are  used.  Let  all  students,  therefore,  embrace  every  favorable  opportunity  for 
looking  up  the  constellations. 

Those  who  wish  to  study  their  mythological  history  will  consult  the  author's  edition 
of  the  "  Geography  of  the  Heavens,'1'1  by  E.  II.  Burritt — the  most  reliable  and  popular 
work  upon  this  subject  in  the  English  language. 

356.  Of  the  nature  and  origin  of  the  constellations 
we  have  already  spoken,  at  343.     Their  formation  has 
been  the  work  of  ages.     Some  of  them  were  known  at 
least  3,000  years  ago.     In  the  9th  chapter  of  Job,  we 

355.  Principal  design  of  this  text-book  ?     What  further  object?    What 
clone  for  this  purpose'?    (Substance  of  note?) 

356.  What  said  of  the  formation  of  the  constellations  ?     Antiquity  ? 
Scripture  .-illusions? 


DESCRIPTION   OF   THE   CONSTELLATIONS.  173 


re/id  of  "  Arcturus,  Orion,  and  Pleiades,  and  the  cham- 
bers of  the  south  ;"  and  in  the  38th  chapter  of  the  same 
book,  it  is  asked,  "  Canst  thou  bind  the  sweet  influences 
of  Pleiades,  or  loose  the  bands  of  Orion  ?  Canst  thou 
bring  forth  Mazzaroth  in  his  season  ?  or  canst  thou  guide 
Arcturus  with  his  sons  ?" 

357.  The  constellations  are  divided  into  ancient  and 
modern.    According  to  Ptolemy's  catalogue,  the  ancients 
had  only  48  constellations ;  but  being  found  convenient 
in  the  study  of  the  heavens,  new  ones  were  added  to 
the  list,  composed  of  stars  not  yet  made  up  into  hydras 
and  dragons,  till  there  a^e  now  scarcely  stars  or  room 
enough  left  to  construct  the  smallest  new  constellation, 
in   all   the   spacious  heavens.     The  present  number, 
according  to  the  catalogue  of  the  Observatory  Eoyal  of 
Paris,  is  93. 

358.  The  constellations  are  further  divided  into  the 
Zodiacal,  Northern,  and  Southern.     The  zodiacal  con- 
stellations are  those  which  lie  in  the  sun's  apparent  path, 
or  along  the  line  of  the  zodiac.     The  northern  are  those 
which  are  situated  between  the  zodiacal  and  the  north 
pole  of  the  heavens ;  and  the  southern,  those  which  lie 
between  the  zodiacal  and  the  south  pole  of  the  heavens. 
They  are  distributed  as  follows — viz.,  12  zodiacal,  35 
northern,  and  46  southern. 

This  division  is  convenient  for  reference ;  but  in  tracing  the  constellations  in  the 
heavens,  or  upon  a  map,  it  is  better  to  begin  with  those  that  are  on  or  near  the  meridian, 
mid  proceed  eastward,  taking  northern  and  southern  together,  so  far  as  they  are  in  view. 
And  where  classes  in  astronomy  are  organized  during  "the  fall  months,  it  will  be  found 
advantageous  to  begin  with  the  constellations  that  are  in  view  at  seasonable  hours  during 
those  months. 

359.  In  consequence  of  the  eastward  motion  of  the 
earth  in  its  annual  revolution,  the  constellations  rise  ear- 
lier and  earlier  every  night ;  so  that  if  an  observer  were 
to  watch  the  stars  from  the  same  position  for  a  whole 
year,  he  would  see  each  constellation,  in  turn,  coming  to 
the  meridian  at  midnight  (or  at  any  other  hour  fixed 

357.  How  are  the  constellations  classified  ?    How  many  of  eacn  ?    In  ull  ? 

358.  How  further  classified  ?    Describe  each.    How  many  of  each  ?    (Whut 
Baid  in  note  ?) 

359.  What  said  of  the  rising  of  the  constellations?    How  proceed  in  de- 
scribing aud  tracing  ? 


174:  ASTRONOMY. 


upon),  till  he  had  seen  the  whole  panorama  of  the  heav- 
ens. Beginning,  therefore,  with  the  constellations  that 
are  on  or  near  the  meridian  at  9  o'clock,  on  the  15th  ot 
November,  and  going  eastward,  we  shall  now  proceed 
with  our  description  of  the  constellations. 

OCTOBER,    NOVEMBER,    AND  DECEMBER. 

360.  ANDROMEDA. — Almost  directly  over  head,  at  9 
o'clock,  on  the  15th  of  November,  may  be  seen  the  con- 
stellation Andromeda.     The  figure  is  that  of  a  woman 
in  a  sitting  posture,  with   her  head   to  the  southv/est. 
Andromeda  may  be  known  by  three  stars  of  the  second 
magnitude,  situated  about  12°  apart,  nearly  in  a  straight 
line,  and  extending  from  east  to  west.     The  middle  star 
of  the  three  is  situated  in  her  girdle,  and  is  called 
Mirach.     The  one  west  of  Mirach  is  Alplieratz,  in  the 
head  of  Andromeda;  and  the  eastern  one,  called  Al- 
maak,  is  in  her  left  foot.     The  star  in  her  head  is  in  the 
equinoctial  colure.     The  three  largest  stars  in  this  con- 
stellation are  of  the  second  magnitude.     Near  Mirach, 
are  two  stars  of  the  third  and  fourth  magnitudes,  and  the 
three  in  a  row  constitute  the  girdle. 

This  constellation  embraces  66  stars,  of  which  three  are  of  the  2d  magnitude,  two  of 
the  3d,  and  the  rest  small.  About  2°  from  v,  at  the  northwestern  extremity  of  tho 
girdle,  is  a  remarkable  cluster  or  nebula  of  very  minute  stars,  and  the  only  one  of  the 
kind  which  is  ever  visible  to  the  naked  eye.  It  resembles  two  cones  of  light,  joined  ot 
their  base,  about  (j°  in  length,  and  ±°  in  breadth. 

361.  PEGASUS  (the  Flying  Horse}. — The  figure  is  the 
head  and  fore  parts  of  a  horse,  with  wings.     The  three 
principal  stars  are  of  the  2d  magnitude — viz.,  Algenib, 
about  15°  south  of  Alpheratz,  in  Andromeda ;  Markab, 
about  18°  west  of  Algenib;    and  8Jceat,\5°  north  of 
Markab.     These  three,  with  Alpherat  in  Andromeda, 
form  what  is  called  the  Square  of  Pegasus.     The  head 
of  the  figure  is  to  the  southwest,  almost  in  a  line  with 
Algenib  and  Markab,  and  about  20°  from  the  latter. 

360.  Constellations  on  the  meridian,  in  what  months  taken  up  ?  An, 
dromeda — where  situated  ?  Figure  ?  Position  ?  How  known  ?  F-mie 
principal  stars.  (How  many  stars  in  constellation?-  What  cluster,  and 
where  ?) 

861.  Figure  of  Pegasus?  Principal  stars?  How  situated?  Fonr'ig 
what  ?  How  the  horse  situated  ?  His  head  where 


DESCRIPTION   OF  THE   CONSTELLATIONS.  175 


362.  PISCES  (the  Fishes)  consists  of  two  fishes,  distin- 
guished as  the  northern  and  western,  connected  by  an 
irregular  line  of  stars. 

The  Western  Fish  is  situated  directly  south  of  the 
square  of  Pegasus — is  about  20°  long,  with  its  head  to 
the  west.  It  includes  a  number  of  small  stars,  just 
south  of  Pegasus. 

The  Northern  Fish  is  about  the  same  size,  with  its 
head  near  Mirach  in  Andromeda,  and  its  body  extending 
to  the  south.  This,  also,  includes  small  stars  only,  and 
is  by  no  means  conspicuous. 

The  flexure  or  ribbon,  uniting  the  tails  of  the  northern 
and  western  fishes,  extends  eastward  from  the  latter,  from 
star  to  star,  till  it  comes  opposite  the  former,  when  it 
turns  to  the  north,  taking  several  small  stars  in  its  way, 
till  it  joins  the  northern  fish. 

363.  AQUARIUS  (the  Water-bearer)  is  represented  by 
the  figure  of  a  man  in  a  reclining  posture,  with  his  head 
to  the  northwest.     Its  four  largest  stars  are  of  the  third 
magnitude.     It  is  situated  directly  south  of  the  head  of 
Pegasus,  and  from  5°  to  30°  north  of  a  star  of  the  first 
magnitude,  in  the  southern  fish.     Three  of  the  principal 
stars  of  Aquarius  are  near  each  other  in  the  water-pot 
which  he  holds  in  his  right  hand. 

364.  PISCIS  AUSTRALIS  (the  Southern  Fish)  is  situated 
directly  south  of  Aquarius.     Its  largest  star  is  Fomal- 
haut,  of  the  1st  magnitude,  which  constitutes  the  eye  of 
the  fish.     The  body  extends  westward  about  20°. 

365.  GRUS  (the  Crane)  is  situated  directly  south  of  the 
southern  fish,  with  its  head  to  the  north.     It  is  composed 
of  a  few  stars  only,  of  the  fourth  magnitude.     As  it  is 
45°  south  of  the  equinoctial,  it  appears  low  down  in  the 
south  to  persons  situated  in  the  Middle  or  Eastern  States. 

366.  THE  PHCENIX  is  about  25°  east  of  the  Crane.     It 

862.  Describe  Pisces.    The  Western  Fish  ?    The  Northern  ?    Flexure  ? 
363.  Figure  of  Aquarius  f    Largest  stars  1    Situation  and  extent  I    Fur- 
ther description. 

Stj-L  Pisces  Aastralis — largest  star?    Situation  of  figure  ? 
865.  Gru& — 'how  situated  ?     Where  ?     Composition  ( 
3(J6.  Situation  of  the  Phcenix  f    Principal  stun*  ? 


17G  ASTRONOMY. 


has  two  stars  of  the  2d  magnitude,  about  12°  apart  east 
and  west.  The  most  western  of  these,  in  the  neck  of  the 
bird,  is  about  25°  southeast  of  Fomalhaut,  in  the  South- 
ern Fish.  The  other  stars  of  the  figure  are  of  the  3d 
and  4th  magnitudes. 

367.  CASSIOPEIA  (the  Queen). — About  30°  northeast  of 
Andromeda  is  Cassiopeia.     The  figure  is  that  of  a  woman 
sitting  in  a  chair,  with  her  head  from  the  pole,  and  her 
body  in  the  Milky  Way.     Its  four  largest  stars  are  of 
the  3d  magnitude. 

368.  PERSEUS   (the    King). — Directly   north    of    the 
"  seven  stars,"  and  east  of  Andromeda,  is  Perseus.    The 
figure  is  that  of  a  man  with  a  sword  in  his  right  hand, 
arid  the  head  of  Medusa  in  his  left.     Algol,  a  star  of  the 
2d  magnitude,  is  about  18°  from  the  Pleiades  (or  seven 
stars),  in  the  head  of  Medusa  ;  and  9°   northeast  of  Al- 
gol is  Algenib,  of  the  same  magnitude,  in  the  back  of 
Perseus.     It  embraces  four  other  stars  of  the  third  mag- 
nitude, besides  many  smaller. 

369.  MUSCA  (the  Fly)  is  about  12°  south  of  Medusa's 
head.     It  is  a  very  small  constellation,  embracing  one 
star  of  the  2d  magnitude,  two  of  the  3d,  and  a  few 
smaller. 

370.  THE  TRIANGLES  include  a  few  small  stars,  about 
half-way  between  Musca  in  the  southeast,  and  Mirach  in 
Andromeda  in  the  northwest.     Its  two  principal  stars 
are  of  the  3d  magnitude. 

371.  ARIES  (the  Ram). — The  head  of  Aries  is  about 
10°  south  of  the  Triangles.     It  may  be  known  by  two 
stars  about  4°  apart,  of  the  3d  and  4th  magnitudes.    The 
most  northeasterly  of  the  two  is  the  brightest,  and  is 
called  a  Arietis.    The  back  of  the  figure  is  to  the  north, 
and  the  body  extends  eastward  almost  to  the  Pleiades. ' 

367.  Where  is  Cassiopeia  f    Fisrure  ?    Situation?    Largest  stars  ? 

368.  Perseus — figure  ?    Two  principal  stars  ?    Names  ?    Situation  ?    Mag- 
nitude ? 

369.  Where  is  Musca  ?    Size  ?    Composition  ? 

370.  The  Triangles — where  ?    Principal  stars  ? 

871.  Where  is  Aries  f     How  known?     Which  of  two  principal 
brightest  ?    Name  ?    How  figure  situated  ?    Extent  3 


DESCRIPTION   OF   THE   CONSTELLATIONS.  177 


372.  CETUS  (the  Whale). — Directly  southeast  of  Ari- 
etis,  and  about  25°  distant,  is  Menkar,  a  star  of  the  2d 
magnitude,  in  the  mouth  of  Cetus.     This  is  the  largest 
constellation  in  the  heavens.     It  is  situated  below  01 
south  of  Aries.     It  is  represented  with  its  head  to  the 
east,  and  extends  50°  east  and  west,  with  an  average 
breadth  of  20°.     The  head  of  Cetus  may  be  known  by 
five  remarkable  stars,  4°  and  5°  apart,  and  so  situated  as 
to  form  a  regular  pentagon,  or  five-sided  figure.     About 
400  southwest  of  Menkar,  is  another  star  in  the  body  ot 
the  figure,  near  which  are  four  small  stars  nearly  in  a 
row,  and  close  together,  running  east  and  west. 

Passing  eastward,  we  next  take  the  constellations  that 
are  on  the  meridian  in 

JANUARY,    FEBRUARY,    AND   MARCH. 

373.  TAURUS  (the  Bull)  will  be  readily  found  by  the 
seven  stars  or  Pleiades,  which  lie  in  his  neck.    The 
largest  star  in  Taurus  is  Aldebaran,  in  the  Bull's  eye,  a 
star  of  the  first  magnitude,  of  a  reddish  color,  somewhat 
resembling  the  planet  Mars.     Aldebaran,  and  four  other 
stars  in  the  face  of  Taurus,  compose  the  Hyades.    They 
are  so  placed  as  to  form  the  letter  Y. 

374.  ORION  lies  southeast  of  Taurus,  and  is  one  of  the 
most  conspicuous  and  beautiful  of  the  constellations.  The 
figure  is  that  of  a  man  in  the  act  of  assaulting  the  bull, 
with  a  sword  in  his  belt,  and  a  club  in  his  right  hand. 
It  contains  two  stars  of  the  first  magnitude,  four  of  the 
second,  three  of  the  third,  and  fifteen  of  the  fourth.     Be- 
telguese  forms  the  right,  and  Bellatrix  the  left  shoulder. 
A  loose  cluster  of  small  stars  forms  the  head.     Three 
small  stars,  forming  a  straight  line  about  3°  in  length, 
constitute  the  lelt,  called  by  Job  "  the  lands  of  Orion" 
They  are  sometimes  called  the  Three  Kings,  because 
they  point  out  the  Hyades  and  Pleiades  on  the  one  hand. 

372.  Cetus — what  star  pointed  out  ?     Size  of  constellation  ?     Situation 
Extent?    How  know  its  head?    "What  other  star  pointed  out ?    "Whutcvm- 
Btellutions  next  described  in  order  ? 

37;3.  Taurus — how  found  ?     Largest  star  ?     Hyades? 

374.   O'-ion,— situation  I     (Jhui-uotur  I     i'Ujuro  I     Composition  ? 


178  ASTRONOMY. 


and  Sinus  on  the  other.  A  row  of  very  small  stars  runs 
down  from  the  belt,  forming  the  sword.  These,  with  the 
stars  of  the  belt,  are  sometimes  called  the  Ell  and 
Yard.  Mintaka,  the  northernmost  star  in  the  belt,  is 
less  than  J°  south  of  the  equinoctial.  Rigel,  a  bright 
star  of  the  first  magnitude,  is  in  the  left  foot,  15°  south 
of  Bellatrix  ;  and  Saiph,  of  the  third  magnitude,  is  situ- 
ated in  the  right  knee,  8J°  east  of  Rigel. 

375.  LEPUS  (the  Hare)  is  directly  south  of  and  near 
Orion.     It  may  be  known  by  four  stars  of  the  third  mag- 
nitude, in  the  form  of  an  irregular  square.     Zeta,  of  the 
fourth  magnitude,  is  the  first  star,  situated  in  the  back, 
and  about  5°  south  of  Saiph  in  Orion.     About  the  same 
distance  below  Zeta  are  the  four  principal  stars,  in  the 
legs  and  feet. 

376.  COLUMBA  NOACHI  (Noah's  Dove)  lies  about  16° 
south  of  Lepus.     It  contains  but-  four  stars,  of  which 
Phaet  is  the  brightest.     It  lies  on  the  right,  a  little 
higher  than  Beta,  the  next  brightest.     This  last  may  be 
known  by  a  small  star  just  east  of  it. 

377.  EKIDANUS  (the  River  Po)  is  a  large  and  irregular 
constellation,  very  difficult  to  trace.     It  is  130°  in  length, 
and  is  divided  into  the  northern  and  southern  streams. 
The  former  lies  between  Orion  and  Cetus,  commencing 
near  Kigel  in  the  foot  of  Orion,  and  flowing  out  westerly 
in  a  serpentine  course,  near  40°,  to  the  Whale. 

378.  CANIS  MAJOR  (the  Greater  Dog)  lies  southeast  of 
Orion,  and  may  be  readily  found  by  the  brilliancy  of  its 
principal  star,  Sirius.    This  is  the  largest  of  the  fixed 
stars,  and  was  once  supposed  to  be  the  nearest  to  the 
solar  system.     Several  others  are  now  supposed  to  be 
nearer. 

879.  ARGO  NAVIS  (the  Ship  Argo)  is  a  large  and 
splendid  constellation  southeast  of  Sirius,  but  so  low  down 
in  the  south  that  but  little  of  it  can  be  seen  in  the  United 


875.  Where  is  Lepus?    How  known?    Describe. 

876.  Cbktmba  Noachi — situation  ?    Composition  ? 

877.  Describe  Eridanus.    Length?    Division?    Situation? 

873.  Where  is  Canis  Major  situated  ?    How  found  ?    What  of  Sit 

879.  Describe  Argo  Xusis.    Where  situated?    Principal  stum  and  where? 


DESCRIPTION   OF   THE   CONSTELLATIONS.  179 


States.  It  lies  southeast  of  Canis  Major,  and  may  be 
known  bj  the  stars  in  the  prow  of  the  ship.  Harkeb,  of 
the  fourth  magnitude,  is  16°  southeast  of  Sirius.  Naas 
and  7,  still  further  south,  are  of  the  second  magnitude, 
and  Canopus  and  Miaplacidus  of  the  first. 

380.  CANIS  MINOR  (the  Lesser  Dog)  is  situated  about 
25°  northeast  of  Sirius,  and  between  Canis  Major  and 
Cancer.     It  is  a  small   constellation,  having  one  star, 
Procyon,  of  the  1st  magnitude,  and  Gomelza,  of  the  2d. 

381.  MONOCEKOS  (the  Unicorn). — A  little  more  than 
half  way  from  Procyon  to  Betelguese  in  Orion,  are  three 
stars  in  a  row,  about  4°  apart,  and  of  the  4th  magnitude. 
They  extend  from  northeast  to  southwest,  and  constitute 
the  face  of  Monoceros.     His  head  is  to  the  west,  with 
Canis  Minor  on  his  back,  and  his  hind  feet  about  25° 
southeast  of  Procyon.     It  is  a  large  constellation,  with 
but  few  stars,  and  those  mostly  small. 

382.  HYDRA  (the  Water  Serpent).— About  20°  east  of 
Procyon  are  four  stars  of  the  fourth  magnitude,  situated 
about  4°  apart,  and  so  as  to  form  a  diamond;  the  longer 
axis  running  east  and  west.     These  constitute  the  head 
of  Hydra,,  which  points  to  the  west.     The  figure  extends 
to  the  south  and  east  more  than  100°,  taking  in  an  ir- 
regular line  of  stars  of  the  3d  and  4th  magnitudes.    The 
largest  star  is  about  15°  southeast  of  the  head.     It  is  of 
the  2d  magnitude,  and  is  called  AlpJiard. 

383.  CANCER  (the  Crab)  is  the  least  remarkable  of  the 
zodiacal  constellations.     It  is  situated  about  15°  north  of 
the  diamond  in  Hydra.     It  has  no  stars  larger  than  the 
3d  magnitude,  and  is  distinguished  for  a  group  of  small 
stars  called  the  Nebula  of  Cancer,  which  is  often  mis- 
taken, for  a  comet.     A  common  telescope  resolves  this 
nebula  into  a  beautiful  assemblage  of  bright  stars. 

384.  GEMINI  (the  Twins)  may  be  known  by  two  bright 

380.  Cam*  Minor — where  ?    Describe. 

881.  Where  is  Monoceros?    How  situated?    Composed?    Character? 

882.  Where  is  the  head  of  Hydra?    How  formed  \    Extent  and  position  ? 
Largest,  star  ? 

883.  Describe   Cancer,      Situation?      Composition?      For  what  distiu- 


1  SO  ASTRONOMY. 


stars  of  the  2d  magnitude — one  in  the  head  of  each 
figure.  They  are  about  5°  apart ;  the  northeasterly  one, 
and  the  brightest  of  the  two,  being  about  25°  due  north 
ofProcyon.  This  ia  PottuM  j  and  the  other  one  is  called 
Castor.  The  bodies  of  the  Twins  extend  from  Castor 
and  Pollux  about  15°  to  the  southwest,  or  toward  Betel- 
guese,  in  the  right  shoulder  of  Orion. 

"  This  constellation,"  says  Dr.  Adam  Clark,  "  was  deemed  propitious  to  mariners ;" 
and  on  this  account,  the  ship  in  which  St  Paul  sailed  from  Alexandria  (Acts  xxviii.  11) 
had  the  sign  of  Castor  and  Poll'ix. 

385.  HERSCHEL'S  TELESCOPE  covers  two  stars  of  the  5th 
magnitude,  near  each  other,  and  about  10°  north  of  Cas- 
tor ;  and  one  other  star  of  the  same  magnitude,  about 
10°  northwest  of  the  two  first  named.     It  is  a  small  aftair 
to  immortalize  Herschel's  grand  telescope. 

386.  THE  LYNX  is  situated  between  Gemini  and  Can- 
cer on  the  south,  and  the  Pole  in  the  north,  the  head 
being  to  the  northwest.     It  has  no  stars  larger  than  the 
4th  magnitude,  and  these  are  in  two  pairs — the  first  15° 
northeast  of  Cancer,  and  the  other  30°  north  of  it.     It  is 
a  loose  and  tame  constellation,  with  nothing  striking  or 
peculiar  by  which  it  may  be  identified. 

387.  CAMELOPAKDALUS  (the  Camelopard)  extends  from 
Perseus  to  the  Pole.     This,  too,  is  a  tame  and  uninterest- 
ing constellation,  with  but  few  stars  in  it,  and  those  of 
the  4th  magnitude,  or  less.     The  hind  feet  of  the  figure 
touch  the  llilky  Way,  and  the  head  is  composed  of  two 
stars  of  the  5th  magnitude,  5°  and  10°  from  the  Pole 
star,  toward  the  "  dipper"  in  the  Great  Bear. 

We  now  pass  eastward  to  constellations  that  are  on  the 
meridian  in 

APRIL,   MAY,   AND   JUNE. 

388.  URSA  MAJOR  (the  Great  Bear)  is  one  of  the  most 
conspicuous  in  the  northern  heavens.     It  may  be  known 

384.  Gemini — how  known  ?    Names  and  situation  of  principal  stars  ?    Of 
figures?    (Note.) 

385.  fferschel's  Telescope — where?    Character? 

886.  Situation  of  the  Lynx  ?    Position  ?    Character  ? 

887.  Position  of  CameUpardalis  t    Extent?    Character?    Where  the  feet  ? 
Tlie  head,  uud  how  composed  ?     What  range  of  constellations  next  do- 
&cribe<J  * 


DESCRIPTION   OF   THE   CONSTELLATIONS.  181 


by  the  figure  of  a  large  dipper,  which  constitutes  the 
ninder  part  of  the  animal.  This  dipper  is  composed  of 
seven  stars.  The  first,  in  the  end  of  the  handle,  is  called 
Benetnash,  and  is  of  the  2d  magnitude.  The  next  is 
Mizar,  known  by  a  minute  star  almost  touching  it,  called 
Alcor.  Mizar  is  a  double  star.  The  third  in  the  handle 
is  Alioth.  The  first  star  in  the  bowl  of  the  dipper,  at 
the  junction  of  the  handle,  is  Megrez.  Passing  to  the 
bottom  of  the  dipper,  we  find  Phad  and  Merak,  while 
Dub/ie  forms  the  rim  opposite  the  handle.  Merak  and 
Dubhe  are  called  the  Pointers,  because  they  always  point 
toward  the  Pole  star.  The  head  of  the  Great  Bear  lies 
far  to  the  west  of  the  Pointers  (apparently  east  when 
seen  below  the  Pole),  and  is  composed  of  numerous  small 
stars  ;  while  \hefeet  are  severally  composed  of  two  small 
stars,  very  near  to  each  other.  Megrez  in  Ursa  Major, 
and  Caph  in  Cassiopeia,  are  almost  exactly  opposite  each 
other,  on  different  sides  of  the  Pole  star,  and  about 
equally  distant  from  it.  They  are  both  in  the  equinoc- 
tial colure. 

389.  LEO  (the  Lion). — About  55°  southwest  of  the 
Pointers  is  liegulus,  a  star  of  the  1st  magnitude,  in  the 
breast  of  Leo.     This   star  is  situated  directly  in  the 
ecliptic.     The  head  of  the  figure  is  to  the  west,  the  back 
being  to  the  south.     North  of  Regulus  are  several  bright 
stars,  in  the  form  of  a  sickle,  of  which  Regulus  is  the 
handle.     Denebola  is  a  bright  star  of  the  2d  magnitude, 
in  the  Lion's  tail,  about  25°  northeast  of  Regulus,  and 
35°  west  of  Arcturus. 

390.  LEO  MINOR  (the  Lesser  Lion)  is  a  small  cluster 
of  stars,  of  which  one  is  of  the  3d,  and  others  of  the  5th 
magnitude,  about  half  way  between  Regulus  and  the 
Pointers.     The  head  of  the  figure  is  northwest,  and  the 
principal  stars  form  the  body  in  the  east,  and  the  fore 
paws,  which  are  extended  to  the  west. 

388.  Describe  Ursa  Majm:  How  known  ?  Names  of  principal  stars  ? 
Which  are  the  Pointers  ?  What  said  of  Megrez  and  CopJi  ? 

3S9.  Where  is  Regulus?  In  what  constellation  ?  How  situated  ?  Magni- 
tude? How  Leo  placed ?  Where  is  the  sickle ?  How  constituted  ?  "Where 
is  Dene1  old  ? 

S'JO.  Describe  Leo  Mln^r.     Where  p,nd  how  situated  ? 

16 


182  ASTRONOMY. 


391.  COMA  BERENICES  (Berenices'  Hair)  is  a  beautiful 
cluster  of  small  stars,  about  20°  northeast  of  Denebola, 
and  half  way  from  Leo  Minor  to  Arcturus.     It  has  but 
one  star  as  large  as  the  4th  magnitude. 

392.  COR  CAROLI  (Charles's  Heart)  is  a  bright  star  of 
the  3d  magnitude,  about  12°  north  of  Coma  Berenices. 
The  figure  includes  several  other  stars,  east  and  west,  of 
the  5th  magnitude. 

393.  BOOTES  (the  Bear-driver)  is  directly  east  of  Coma 
Berenices.     The  figure  is  that  of  a  man,  with  his  head 
toward  the  Pole,  and  Arcturus,  a  star  of  the  1st  magni- 
tude, in  the  left  knee.    The  other  stars  are  of  the  3d  and 
4th  magnitudes.     Three  small  stars,  forming  a  triangle, 
and  situated  15°  northeast  of  Arcturus,  mark  the  right 
hand  of  the  figure ;  while  two  stars  of  the  3d  and  4th 
magnitudes,  and  still  further  north,  mark  his  shoulders. 
The  head  is  marked  by  Nekkar,  another  star  of  the  3d 
magnitude. 

394.  VIRGO  (the  Virgin)  lies  directly  south  of  Coma 
Berenices  and  Bootes.     The  figure  is  that  of  a  woman 
with  wings,  with  her  head  to  the  west,  near  Denebola  in 
Leo ;  and  her  feet  about  40°  to  the  east.     Spica,  the  prin- 
cipal star,  is  of  the  1st  magnitude,  about  35°  southwest 
of  Arcturus. 

395.  CRATER  (the  Cup)  is  composed  of  six  small  stars 
30°  west  of  Spica.    The  largest  is  of  the  4th  magnitude. 

396.  CoRvus(the  Crow)  is  still  nearer,  being  only  15° 
southwest  of  Spica.     It  has  two  stars  of  the  3d  magni- 
tude, and  three  of  the  4th. 

397.  LIBRA  (the  Balance)  is  about  25°  east  of  Spica. 
It  has  two  stars  of  the  2d  magnitude,  about  10°  apart, 
which,  with  two  others  of  the  3d  magnitude  southeast  of 

891.  Coma  Berenices — character?    Situation? 

892.  Cor  Goruli — principal  star  ?     Situation  ? 

893.  Where  is  Bootes?    Figure  ?    Position  ?    Principal  stars  ? 

894.  Where  is  Virgo  situated  ?    Figure  ?    Position  ?    Principal  stars  ? 
395.  Crater — how  situated  ?    Largest  star  ? 

896.  Corvis — where  ?    Composition  ? 

897.  Where  is  Libra,  f    Composition  ? 


DESCRIPTION   OF  THE   CONSTELLATIONS.  183 


them,  form  a  small  quadrilateral  figure.     Its  few  remain- 
ing stars  are  at  the  east,  and  of  the  4th  magnitude. 

398.  CENTAURUS  (the  Centaur)  is  a  fine  compact  con- 
stellation about  30°  south  or  southeast  of  Spica.     It  has 
nine  stars  of  the  3d  magnitude,  mostly  in  the  head  of 
the  figure.     It  is  too  low  in  the  south  to  be  visible  in  the 
United  States,  except  when  near  the  meridian. 

JULY,   AUGUST,   AND  SEPTEMBER. 

399.  URSA  MINOR  (the  Lesser  Bear)  is  composed  of  a 
few  stars  near  the  north  pole  of  the  heavens,  and  mostly 
of  the  3d  and  4th  magnitudes.     The  back  of  the  figure 
is  toward  the  pole,  with  its  head  to  the  west.    The  Pole 
star^  of  the  2d  magnitude,  is  in  the  extremity  of  its  tail. 

400.  DRACO  (the  Dragon)  is  an  irregular  serpentine  con 
Btellation,  embracing  a  large  circuit  in  the  polar  regions. 
He  winds  round  between  the  Great  and  Little  Bear,  and, 
commencing  with  the  tail,  between  the  Pointers  and  Pole 
star,  is  easily  traced,  by  a  succession  of  bright  stars 
extending  from  west  to   east.     Passing  south  of  Ursa 
Minor,  around  nearly  to  Cepheus,  it  returns  westward, 
and  terminates  in  four  stars,  which  form  the  head,  near 
the  foot  of  Hercules.     These  four  stars  are  3°,  4°,  and  5° 
apart,  so  situated  as  to  form  an  irregular  square ;  the  two 
upper  ones,  Etamis  and  Rastdben,  being  the  brightest, 
and  both  of  the  2d  magnitude. 

401.  HERCULES  (the  Giant)  is  a  large,  but  not  very 
striking  or  conspicuous  constellation.     The  figure  is  that 
of  a  giant,  with  a  large  club  in  his  right  hand,  and  a 
hydra  in  his  left.     The  head  of  the  figure  is  to  the  south, 
and  the  whole  is  composed  of  stars  from  the  2d  to  the  4th 
magnitude.     This  constellation  is  thickly  set  with  stare, 
the  largest  of  which  is  called  Rasalgethi,  in  the  head 

398.  Describe  Centaurus.    Position?    Composition? 

399.  Urta Minor — position?    Principal  star 2 

400.  Draco— position?     How  traced?     Where  head?     How  composed? 
Form  what  ? 

401.  Hercules— figure?    Situation?    Composition?    Principal  star  ?    Num- 
oer  of  stars  ? 


18-1  ASTRONOMY. 


ot  the  figure,  and  is  of  the  2d  magnitude.     It  has  nine 
stars  of  the  3d  magnitude,  and  19  of  the  4th. 

402.  CORONA  BOREALIS  (the  Northern  Crown)  is  about 
15°  west  of  the  middle  of  Hercules.     Its  principal  star 
is  Alphacca^  a  bright  star  of  the  2d  magnitude,  about 
20°  northeast  of  Arcturus.     About  the  same  distance, 
directly  east  of  Arcturus,  is  a  small  group  of  stars,  which 
constitute  the  head  of  the  Serpent. 

403.  SCORPIO  (the  Scorpion)  is  one  of  the  most  interest 
ing  and  splendid  of  the  constellations.     It  is  situated 
about  45°  east  of  Spica,  adjoining  Libra.     The  head  of 
the  figure  is  composed  of  five  stars — one  of  the  2d,  and 
the  others  of  the  3d  magnitude — forming  an  arc  of  a  cir- 
cle convex  to  the  west.     The  largest  of  these  five  stars  is 
in  the  ecliptic,  and  is  called  Graffias.     About  9°  south- 
east of  Graffias  is  Antares,  a  star  of  the  1st  magnitude, 
in  the  body  of  the  figure,  and  of  a  reddish  color.     A 
number  of  bright  stars  of  the  4th  magnitude  extend  to 
the  southeast  into  the  Milky  Way,  and  then  curve  around 
to  the  east  and  north,  forming  the  tail  of  Scorpio. 

404.  LUPUS  (the  "Wolf)  consists  of  a  small  group  of 
stars,  about  15°  southwest  of  Antares.     The  head  of  the 
figure  is  to  the  north. 

405.  SERPENTARIUS  (the  Serpent-bearer)  is  a  large  but 
uninteresting  constellation,  between  Scorpio  on  the  south, 
and  Hercules  on  the  north.     The  figure  is  that  of  a  man 
grasping  a  serpent,  the  head  of  which  has  already  been 
described  (402).     The  folds  of  the  serpent  may  be  traced 
by  a  succession  of  bright  stars  extending  for  some  dis- 
tance to  the  east.     The  principal  star  in  Serpentarius  is  oi 
the  2d  magnitude,  and  is  called  JKas  Alhayue.     It  is 
situated  in  the  head  of  the  figure,  and  within  5°  of  Ea- 
ealgethi,  in  the  head  of  Hercules.     The  feet  of  the  figure 


402.  Coroni  Borealis — location?    Principal   star?    "What  other  group  of 
stars  mentioned  ? 

403.  Describe  Scorpio.    Situation?    Composition?    Largest  star- in  head  • 
What  other  large  star?    Position  and  composition  of  tail  ? 

404.  Lepus — composition  ?    Position  ? 

40o.  Serpentarius — situation?    Figure?    Principal  star?    Situation? 


DESCRIPTION   OF   THE   CONSTELLATIONS.  1^6 

rest  upon  Scorpio,  and  the  right  shoulder  touches  the 
Milky  Way. 

406.  LYKA  (the  Harp)  is  a  small  constellation  15°  east 
of  Hercules.     Its  principal  star  is  Yega,  of  the  1st  mag- 
nitude, one  of  the  brightest  stars  in  the  northern  hemi- 
sphere.    It  has  two  stars  of  the  3d  magnitude,  and  sev- 
eral others  of  the  4th. 

407.  CYGNUS  (the  Swan)  is  situated  directly  east  of 
Lyra.     Three  bright  stars,  which  lie  along  the  Milky 
Way,  form   the   body  and  neck  of  the  Swan,  running 
northeast  and  southwest ;  and  two  others,  at  right  angles, 
in  a  line  with  the  middle  one  of  the  three,  constitute  the 
wings.    These  five  stars  form  a  large  cross.    Arided,  in 
the  body  of  the  Swan,  is  a  star  of  the  1st  magnitude, 
and  the  remaining  ones  of  the  constellation  are  of  the 
3d  and  4th. 

408.  THE  Fox  AND  GOOSE  is  located  just  south  of  Cyg- 
nus,  with  the  head  to  the  west.     It  is  a  small  constella- 
tion ;  the  two  principal  stars  of  which,  of  the  2d  magni- 
tude, form  the  head  of  the  Fox.     Most  of  the  figure  is 
in  the  Milky  Way. 

409.  AQUILA  (the  Eagie)  is  still  'south  of  Cygnus  and 
the  Fox.     It  is  conspicuous  for  three  bright  stars  in  its 
neck,  of  which  the  central  one,  Altair^  is  a  brilliant 
white  star  of  the  first  magnitude,  just  east  of  the  Galaxy. 

410.  DELPHINUS  (the  Dolphin)  is  a  beautiful  little  clus- 
ter of  stars,  15°  northeast  of  the  Eagle.     It  may  be 
known  by  four  principal  stars  in  the  head,  of  the  3d 
magnitude,  arranged  in  the  figure  of  a  diamond,  and 
pointing  northeast  and  southwest.     A  star  of  the  same 
magnitude,  about  5°  south,  makes  the  tail. 

411.  ANTINOUS  lies  directly  south  of  Aquila,  his  head 
being  near  Altair,  and  the  body  and  feet  to  the  south- 
west.    Two  stars  of  the  3d  magnitude  constitute  the  right 

406.  Lyra — situation  ?    Principal  star  ?    What  others  ? 

407.  Gygnus — situation  ?    Composition  ? 

408.  Fox  and  Goose — location  ?    Position  of  figure  ? 

409.  Aquila — where  ?     For  what  conspicuous  ? 

41 0.  Describe  Delphinus.     How  known  ? 

411.  Antonius — situation  !     How  placed  ?     Composition? 

16* 


180  ASTRONOMY. 


arm,  and  several  smaller  ones  make  the  bow  and  arrows 
held  in  his  hand. 

412.  SAGITTARIUS  (the  Archer)  lies  next  to  Scorpio,  and 
may  be  known  by  three  stars  in  the  Galaxy,  arranged  in 
a  curve,  to  represent  the  bow  of  the  archer.     The  central 
star  is  the  brightest,  and  has  a  bright  star  directly  west 
of  it,  forming  the  head  of  the  arrow.     The  head  and 
chest  of  Sagittarius  are  just  east  of  the  Milky  Way,  be- 
tween the  tail  of  Scorpio  and  the  head  of  Capricornus. 

413.  CAPRICORNUS  (the  Goat)   is  situated  about  20° 
northeast  of  Sagittarius.     The  head  of  the  figure  is  to 
the  west,  and  is  composed  of  two  bright  stars,  of  the  3d 
magnitude,  and  about  4°  apart.     There  is  a  smaller  star 
between  them,  and   several  still  smaller  close  around 
them. 

414.  CRUX  (the  Cross)  is  a  brilliant  little  constellation, 
but  too  far  south  to  be  visible  to  us  at  the  north.     It  con- 
sists of  four  principal  stars — namely,  one  of  the  1st,  two 
of  the  2d,  and  one  of  the  3d  magnitude. 

Besides  these,  there  are  several  fine  constellations  about  the  south  pole  of  the  heav- 
ens, as  the  Altar,  the  Peacock,  Charles's  Oak,  <fcc. ;  but  as  they  cannot  be  traced  from 
the  latitudes  in  which  this  book  will  be  used,  it  is  thought  not  important  to  describe 
them. 


412.  Sagittarius — where  ?    How  known  ? 

413.  Capricornus — where  ?    Position  of  figure  ?    Composition  ? 

414.  Crux — describe.      Composition  ?     (What  said  of  south  clrcumpolar 
eoustellatious  ?    Names !    Why  not  described  ?) 


DOUBLE   STARS.  187 


CHAPTER   III. 

IOUBLE,  VARIABLE,  AND  TEMPORARY  STARS,  BINARY  SYSTEMS,  ETC 

415.  MANY  of  the  stars  which,  to  the  naked  eye,  ap- 
pear single,  are  found,  when  examined  by  the  aid  of  a 
telescope,  to  consist  of  two  or  more  stars,  in  a  state  of 
near  proximity  to  each  other.     These  are  called  Double 
Stars.     When  three  or  more  stars  are  found  thus  closely 
connected,  they  are  called  Triple  or  Multiple  Stars. 

416.  Double  and  triple  stars  are  supposed  to  be  consti- 
tuted  in   two  ways — first,  by  actual   contiguity;   and 
secondly,  where  they  are  only  near  the  same  line  of 
vision,  one  of  the  component  stars  being  far  beyond  the 
other.     In  the  former  case,  they  are  said  to  be  physically 
double,  from  the  belief  that  they  are  bound  together  by 
attraction,  and  that  one  revolves  around  the  other  ;  while 
in  the  latter  case,  they  are  considered  as  only  optically 
double. 

6TAB8  OPTICALLY  DOTTBLB. 

Apparent  position*.  True  position*. 


B 


Here  the  observer  on  the  left  sees  a  large  and  small  star  at  A,  apparently  near  toge- 
ther —  the  lowest  star  being  much  the  smallest  But  instead  of  their  being  situated  as 
they  appear  to  be,  with  respect  to  each  other,  the  true  position  of  the  smaller  star  may 
be  at  B  instead  of  A  ;  and  the  difference  in  their  apparent  magnitudes  may  be  wholly 
owing  to  the  greater  distance  of  the  lower  star. 

Upon  this  subject  Dr.  Herschel  remarks,  that  this  nearness  of  the  stars  to  each  other, 
in  certain  cases,  might  be  attributed  to  some  accidental  cause,  did  it  occur  only  in  a  few 
Instances;  but  the  frequency  of  this  companionship,  the  extreme  closeness,  and,  in 
many  cases,  the  near  equality  of  the  stars  so  conjoined,  would  alone  lead  to  a  strong 
suspicion  of  a  more  near  and  intimate  relation  than  mere  casual  juxtaposition. 

415.  What  said  of  double,  triple,  and  multiple  stars  ? 

416.  How  are  they  supposed  to   be  constituted  ?    How  distinguished  ! 
(Illustrate  by  diagram.     Kemark  of  Dr.  liersckel?    How  many  specimens 
of  double  stars  given  ?) 


1S8  ASTRONOMY. 


The  following  will  convey  to  the  student  an  idea  of  the  telescopic  appearance  of  some 
of  the  double  stars: 

SPECIMENS   OF  DOUBLE   STARS. 


417.  A  is  a  double  star  in  Ursa  Minor,  commonly 
known  as  the  Pole  star.     It  consists  of  a  star  of  the  2d, 
and  another  of  the  9th  magnitude,  situated  about  IS" 
apart,  or  about  four  times  the  diameter  of  the  larger 
star.     They  are  both  of  a  silvery  white.     It  requires  a 
pretty  good  telescope  to  show  this  star  double ;  hence  it 
is  considered  a  very  good  test  object  by  which  to  ascer- 
tain the  qualities  of  a  telescope,  especially  of  the  low- 
priced  refractors. 

The  writer  has  often  seen  the  companion  of  the  Pole  star  distinctly,  with  a  six-inch 
refracting  telescope,  manufactured  by  Mr.  Henry  Fitz,  New  York. 

418.  B  is  a  view  of  the  double  star   Castor,  in  the 
Twins.     The  stars  are  of  a  greenish  white,  of  the  3d  and 
4th  magnitudes,  and  about  5",  or  two  diameters  of  the 
principal  star,  apart.     This  also  is  considered  a  good 
test  object.     Through  ordinary  telescopes,  the  stars  seem 
to  be  in  contact ;  but  with  those  of  higher  power,  they 
appear  fairly  divided.     These  stars  constitute  what  is 
called  a  Binary  System. 

419.  C  is  a  representation  of  Mizar,  the  middle  star, 
in  the  tail  of  the  Great  Bear.     It  may  be  seen  double 
with  a  good  spy-glass.     The  stars  are  both  of  a  greenish 
white,  of  the  3d  and  4th  magnitudes,  and  about  14" 
apart.     Mizar  has  sometimes  been  seen  without  a  com- 
panion, and  at  other  times  it  has  been  known  suddenly 
to  appear.     The  companion  is  not  Alcor,  near  Mizar,  and 
visible  to  the  naked  eye,  but  a  telescopic  star. 

417.  What  is  Fig.  A  in  the  cut  ?    How  composed  ?    Color  ?    How  seen  ! 
(tiemark  of  author  in  note  ?) 

418.  Fig.  B — color?    Magnitudes?    Distance  apart?    Further  remark  ? 

419.  Fig.  C— how  seen ?     Color?     Magnitude!     Distance?     Additional 
remarks  1 


BINARY     AND     OTHER     SYSTEMS.  189 


420.  D  is  a  view  of  the  double  star  Mintaka,  the  mid- 
dle star  of  the  three  forming  the  belt  of  Orion.     The 
component  stars  are  of  the  4th  and  8th  magnitudes — the 
largest  of  a  reddish  hue,  and  the  small  one  white.    They 
are  about  10"  apart,  or  four  times  the  diameter  of  the 
largest  star. 

421.  E  is  a  view  of  Rigel,  in  the  left  foot  of  Orion. 
The  components  are  of  the  1st  and  9th  magnitudes,  and 
about  10"  apart.     Their  color  is  a  yellowish  white. 

422.  F  is  a  view  of  the  bright  star  Vega,  in  the  Lyre. 
Its  companion  is  a  star  of  the  llth  magnitude,  situated 
about  40"  distant.    This  is  a  close  test  object  for  an  ordi- 
nary telescope. 

423.  The  number  of  double  stars  has  been  variously 
estimated.     Sir  William  Herschel  enumerates  upwards 
of  500,  the  individuals  of  which  are  within  30"  of  each 
other.     Professor  Struve  of  Dorpat  estimated  the  num- 
ber at  about  3,000  ;  and  more  recent  observations  fix  the 
number  at  not  less  than  6,000.    The  great  number  of  the 
double  stars  first  led  astronomers  to  suspect  a  physical 
connection  by  the  laws  of  gravitation,  and  also  a  revolu- 
tion of  star  around  star,  as  the  planets  revolve  around 
the  sun. 

BINARY   AND   OTHER   SYSTEMS. 

424.  By  carefully  noting  the  relative  distances  and 
angular  positions  of  double  and  multiple  stars,  for  a  se- 
ries of  years,  it  has  been  found  that  many  of  them  have 
their  periodic  revolutions  around  each  other.     Where 
two  stars  are  found  in  a  state  of  revolution  about  a  com- 
mon center,  they  constitute  what  is  called  a  Binary  Sys- 
tem.    These,  it  must  be  remembered,  are  the  double  and 
multiple  stars,  which  appear  single  to  the  naked  eye. 
Sir  w .  Herschel  noticed  about  50  instances  of  changes 
in  the  angular  position  of  double  stars ;  and  the  revolu 

420.  Fig.  D — describe.    Magnitude  ?    Color  ?    Distance  ? 

421.  Fig.  E — place  \    Components  ?    Distance  ?    Color  ? 

422.  Fig.  F — companion? 

423.  Number  of  double  stars ?    Led  to  what? 

421.  Motions  of  double  stars  ?     What  are  Unary  systems  f 


1 90  ASTRONOMY. 


tion  of  some  eighteen  of  these  is  considered  certain. 
Their  periods  vary  from  40  to  1,200  years. 

425.  The  star  numbered  70  in  the  Serpent-bearer  is  a 
binary  system.     The  periodic  time  of  the  revolving  star 
is  about  93  years.     In  the  course  of  its  revolution,  the 
two  stars  sometimes  appear  separated,  sometimes  very 
near  together,  and  at  other  times  as  one  star.     They  are 
of  the  5th  and  6th  magnitudes,  and  of  a  yellowish  hue. 

426.  The  star  f,  in  the  left  hind  paw  of  Ursa  Major 
is  one  of  these  stellar  systems.     The  revolution  of  its 
component  stars  began  to 'be  noticed   in  1781:  since 
which  time  they  have  made  one  complete  revolution, 
and  are  now  (1853)  some  fourteen  years  on  the  second. 
Of  course,  then,  their  periodic  time  is  about  58  years. 
Their  angular  motion  is  about  6°  24'  per  year. 

Dr.  Dick  supposes  these  stars  to  be  some  200,000,000,000  miles  apart;  and  upon  th< 
supposition  that  tho  smaller  revolves  around  the  latter,  computes  its  velocity  to  be  no* 
less  than  2,471,000  miles  every  hour.  This  would  be  86  times  the  velocity  of  Jupiter 
and  23  times  the  velocity  of  Mercury — the  swiftest  planet  in  the  solar  system. 

427.  The  star  y  in  Yirgo  is  another  of  these  systems. 
It  has  been  known  as  a  double  star  for  at  least  130  years. 
The  two  stars  are  both  of  the  3d  magnitude,  and  of  a 
yellowish  color.     The  late  E.  P.  Mason,  of  Yale  College, 
estimated  its  period  at  171  years.     More  recent  observa 
tions  and  estimates  by  Madler  give  a  period  of  145 
years. 

428.  "To  some  minds,  not  accustomed  to  deep  reflec- 
tion," says  Dr.  Dick,  "  it  may  appear  a  very  trivial  fact 
to  behold  a  small  and  scarcely  distinguishable  point  of 
light  immediately  adjacent  to  a  larger  star,  and  to  be  in- 
formed that  this  lucid  point  revolves  around  its  larger 
attendant ;  but  this  phenomenon,  minute  and  trivial  as 
it  may  at  first  sight  appear,  proclaims  the  astonishing 
fact,  that  suns  revolve  around  suns,  and  systems  around 
systems.     This  is  a  comparative^  new  idea,  derived  from 
our  late  sidereal  investigations,  and  forms  one  of  the 

425.  Describe  70  Opbiuchi? 

426.  What  specimen  described  ?    Period?    Yearly  angular  motion  ?    (Br. 
Dick's  remark  ?) 

427.  What  other  binary  system  ?     How  long  known  ?      Components  1 
Period » 

428.  Quotation  from  Dr.  Dick. 


BINARY      AND     OTHER     SYSTEMS.  191 


most  sublime  conceptions  which  the  modern  discoveries 
of  astronomy  have  imparted. 

429.  "It  undoubtedly  conveys  a  very  sublime  idea, 
to  contemplate  such  a  globe  as  the  planet  Jupiter — a 
body  thirteen  hundred  times  larger  than  the  earth — re- 
volving around  the  sun,  at  the  rate  of  twenty-nine  thou- 
sand miles  every  hour ;  and  the  planet  Saturn,  with  its 
rings  and  moons,  revolving  in  a  similar  manner  round 
this  central  orb,  in  an  orbit  five  thousand  six  hundred 
and  ninety  millions  of  miles  in  circumference.     But  how 
much  more  august  and  overpowering  the  conception  of  a 
sun  revolving  around  another  sun — of  a  sun  encircled 
with  a  retinue  of  huge  planetary  bodies,  all  in  rapid  mo- 
tion, revolving  round  a  distant  sun,  over  a  circumference 
a  hundred  times  larger  than  what  has  been  now  stated, 
and  with  a  velocity  perhaps  a  hundred  times  greater  than 
that  of  either  Jupiter  or  Saturn,  and  carrying  all  its 
planets,  satellites,  comets,  or  other  globes,  along  with  it  in 
its  swift  career !     Such  a  sun,  too,  may  as  far  exceed 
these  planets  in  size  as  our  sun  transcends  in  magnitude 
either  this  earth  or  the  planet  Yenus  ;  the  bulk  of  any 
one  of  which  scarcely  amounts  to  the  thirteen-hundred- 
thousandth  part  of  the  solar  orb  which  enlightens  our 
day. 

430.  "  The  further  we  advance  in  our  explorations  of 
the  distant  regions  of  space,  and  the  more  minute  and 
specific  our  investigations  are,  the  more  august  and  as- 
tonishing are  the  scenes  which  open  to  our  view,  and  the 
more  elevated  do  our  conceptions  become  of  the  gran- 
deur of  that  Almighty  Being  who  '  marshalled  all  the 
starry  hosts,'  and  of  the  multiplicity  and  variety  of  ar- 
rangements he  has  introduced  into  his  vast  creation. 
And  this  consideration  ought  to  serve  as  an  argument 
to  every  rational  being,  both  in  a  scientific  and  a  reli- 
gious point  of  view,  to  stimulate  him  to  a  study  of  the 
operations  of  the  Most  High,  who  is  '  wonderful  in  coun- 
sel, and  excellent  in  working,'  and  whose  works  in  every 

429.  What  further  remarks  ? 

430.  Continue  quotation.    (What  table?    Note?) 


192 


ASTRONOMY. 


part  of  his  dominions  adumbrate  the  glory  of  his  perfec- 
tions, and  proclaim  the  depths  of  his  wisdom,  and  the 
greatness  of  his  power." 

The  following  table  shows  the  periodic  times  of  the 
principal  binary  systems,  so  far  as  known : 


BINARY   SYSTEMS. 


Names. 

Period  in 
years. 

Names. 

Period  in 
years. 

i\  CoronsQ      . 

4:3-4:0 

w  Leonis     .  . 

82-533 

£  Cancri      .  .  . 

55-00 

g  Bootes  

117-140 

\  UrsseMajoris 
70  Ophiuchi  .  .  . 
61  Cygni  
7  Virginia  .  .  . 
Castor  

58-26 
93-00 
452-00 
14:5-00 

286-00 

a  Hercules  .  .  . 
b  Ursge  Majoris 
c      "          " 
p  Ophiuchi  .  .  . 
b          " 

31-468 
58-262 
61-464 
73-862 
80-340 

tf  Coronse  .... 
j  Leonis  

145-00 
1200-00 

c  "  ... 
(3  Coronse  .... 

92-870 
608-450 

The  student  should  here  be  reminded  that  these  are  not  systems  of  planets  revolving 
around  suns,  but  of  sun  revolving  around  #un;  and  that  their  component  stars  may 
not  only  be  as  far  apart  as  our  sun  and  Sirius,  but  that  they  are  probably  each  the  center 
of  his  own  planetary  system,  like  that  which  revolves  around  our  central  orb. 

431.  Besides  the  revolutions  of  these  double  stars 
around  each  other,  they  are  found  to  have  a  proper  mo- 
tion together  in  space,  like  that  which  our  sun  has  around 
the  great  central  Sun.  Upon  this  subject  Sir  John  Her- 
schel  observes,  that  these  stars  not  only  revolve  around 
each  other,  or  about  their  common  center  of  gravity,  but 
that  they  are  also  transferred,  without  parting  company, 
by  a  progressive  motion  common  to  both,  toward  some 
determinate  region. 

The  two  stars  of  61  Cygni,  which  are  nearly  equal,  have  remained  constantly  at  the 
tame,  or  very  nearly  the  same,  distance  of  15",  for  at  least  50  years  past  Meanwhile, 
they  have  shifted  their  local  situation  in  the  heavens,  in  this  interval  of  time,  through 
no  less  than  4'  23" — the  annual  proper  motion  of  each  star  being  5".3;  by  which  quan- 
tity (exceeding  a  third  of  their  interval)  this  system  is  every  year  carried  bodily  along 
in  some  unknown  path,  by  a  motion  which,  for  many  centuries,  must  be  regarded  as 
ULiform  and  rectilinear.  Among  stars  not  double,  and  no  way  differing  from  the  rest  in 
any  other  obvious  particular,  p  Cassiopeiie  is  to  be  remarked  as  having  the  greatest 
proper  motion  of  any  yet  ascertained,  amounting  to  3".74  of  annual  displacement. 

431.  What  other  motion  of  the  stars  ?  Dr.  Herschel  ?  (Specimen  in  note  ? 
Motions  ?  What  star  named  as  having  the  greatest  proper  motion  of  any  vet 
known?)  S 


STARS   OF   VARIOUS   COLORS.  193 


432.  But  though  motions  which  require  whole  centu- 
ries to  accumulate  before  they  produce  changes  of  ar- 
rangement, such  as  the  naked  eye  can  detect,  are  quite 
sufficient  to  destroy  that  idea  of  mathematical  fixity 
which  precludes  speculation,  yet  are  they  too  trifling,  so 
far  as  practical  applications  go,  to  induce  a  change  o* 
language,  and  lead  us  to  speak  of  the  stars,  in  common 
parlance,  as  otherwise  than  fixed. 

433.  Most  of  the  double,  triple,  and  multiple  stars  are 
of  various  colors,  beautifully  contrasting  with  each  other. 


Other  suns,  perhaps, 


"With  their  attendant  moons 
Communicating  male  and  female  light, 
(Which  two  great  sexes  animate  the  world,) 
Stored  in  each  orb,  perhaps,  with  some  that  live." 

It  is  probable,  however,  that,  in  most  cases,  this  variety 
of  colors  is  merely  complimentary,  in  accordance  with 
that  general  law  of  optics  which  provides  that  when  the 
retina  is  under  the  influence  of  excitement,  by  any  bright 
colored  lights,  feebler  lights,  which,  seen  alone,  wouW 
produce  no  sensation  but  of  whiteness,  shall  for  the  time 
appear  colored  with  the  tint  complimentary  to  that  of 
the  brighter.  Thus,  if  a  yellow  color  predominate  in 
the  light  of  the  brighter  star,  that  of  the  less  bright  one 
in  the  same  field  of  view  will  appear  blue  ;  while,  if  the 
tint  of  the  brighter  star  verge  to  crimson,  that  of  the 
other  will  exhibit  a  tendency  to  green,  or  even  appear  as 
a  vivid  green,  under  favorable  circumstances. 

434.  This  first  law  of  contrast  is  beautifully  exhibited 
by  i  Cancri — the  latter  by  7  Andromedse;  both  fine 
double  stars.  If,  however,  the  colored  star  be  much 
the  less  bright  of  the  two,  it  will  not  materially  aflect 
the  other.  Thus,  for  instance,  t\  Cassiopeise  exhibits  the 
beautiful  combination  of  a  large  white  star,  and  a  small 
one  of  a  rich  ruddy  purple. 

It  is  by  no  means,  however,  intended  to  say,  that  in  all  such  cases  one  of  the  colors 
ic  a  mere  efiect  of  contrast ;  and  it  may  be  easier  suggested  in  words  than  conceived  in 

432.  Why  called  "  fixed  stars,"  if  in  motion  ? 

433.  What  said  of  the  color  of  double  stars  ?     Quotation  from  Milton  ? 
Cause  of  this  variety  of  colors  ? 

434.  Specimens  of  complimentary  colors?    ( -ire  they  all  complimentary  ») 

17 


1 01  ASTRONOMY. 


imagination  what  variety  of  illumination  hvo  (nms—a.  red  and  a  green,  or  a  yellow  .w! 
a  blue  one — must  att'ord  a  planet  circulating  about  either,  ai.d  what  charming  contrasts 
and  "  grateful  vicissitudes"1 — a  red  and  a  green  day,  for  instance,  alternating  with  a  white 
one  and  with  darkness— might  arise  from  the  presence  or  absence  of  one  or  other,  or 
both,  above  the  horizon.  Insulated  stars  of  a  red  color,  almost  as  deep  as  that  tit'  blood, 
occur  in  many  parts  of  the  heavens ;  but  no  green  or  blue  star,  of  any  decided  hue,  has, 
we  believe,  ever  been  noticed  unassociated  with  &  companion  brighter  than  itself. 

VARIABLE   OR   PERIODICAL   STARS. 

435.  Variable  stars  are  those  which  undergo  a  regular 
periodical  increase  and  diminution  of  lustre,  amount- 
ing, in  some  cases,  to  a  complete  extinction  and  revival. 
These  variations  of  brilliancy,  to  which  some  of  the 
fixed  stars  are  subject,  are  reckoned  among  the  most 
remarkable  of  celestial  phenomena.     Some  of  them  pass 
through  their  successive  changes  with  great  rapidity  ; 
while,  in  other  cases,  their  brilliancy  is   increased    or 
diminished  gradually  for  months.     The  time  occupied  by 
one  of  these  stars,  in  passing  through  all  their  different 
phases,  is  called  its  period. 

436.  One  of  the  most  remarkable  of  these  variable 
stars  is  the  star  Omicron,  or  Mira  in  the  Whale.     Its 
period  is  about  332  days,  during  which  time  it  varies 
from  a  star  of  the  2d  magnitude  to  complete  invisibility. 
It  appears  about  twelve  times  in  eleven  years — remains 
at  its  greatest  brightness  about  a  fortnight ;  being  then, 
on  some  occasions,  equal  to  a  large  star  of  the  2d  magni- 
tude.    It  then  decreases  for  about  three  months,  when  it 
disappears.     In  about  five  months,  it  becomes  visible 
again,  and  continues  to  increase  during  the  remaining 
three  months  of  its  period. 

Its  increase  of  light  is  much  more  rapid  than  its  de- 
crease. It  increases  from  the  6th  to  the  2d  magnitude 
in  40  days,  continues  thus  brilliant  26  days,  and  then 
fades  to  the  6th  magnitude  again  in  66  days.  Hence  it 
is  above  the  6th  magnitude  for  132  days,  and  below  200 
days  of  its  period. 


435.  What  are  variable  stars  ?    How  regarded  ?    What  difference  ?    What 
their  period  ? 

436.  What  remarkable  sample  described  ?    Period  ?    Amount  of  variation  ? 
Progress  of  variation  ? 


VARIABLE     OB     PERIODICAL     STAK3.  195 


437.  Another  remarkable  periodic  star  is  that  called 
Algol,  in  the  constellation  Perseus.     It  is  usually  visible 
as  a  star  of  the  2d  magnitude,  and  such  it  continues  for 
the  space  of  2  days  14  hours,  when  it  suddenly  begins 
to  diminish  in  splendor ;  and  in  about  3  J  hours,  it  is  re- 
duced to  the  4th  magnitude.     It  then  begins  again  to 
increase,  and  in  3£  hours  more  is  restored  to  its  usual 
brightness  ;  going  through  all  its  changes  in  2  days  20 
hours  and  48  minutes,  or  thereabouts.     Through  all  its 
successive  changes,  this  star  shines  with  a  white  light, 
while  the  color  of  all  the  other  variable  stars  is  red. 

438.  The  cause  of  these   periodic  variations  in  the 
brightness  of  some  of  the  stars  is  not  known.     Borne 
suppose  them  to  be  occasioned  by  opake  bodies  revolv- 
ing around  them,  and  cutting  oif  a  portion  of  their  light 
from  us.     Speaking  of  the  sudden  obscuration  of  Algol, 
mentioned  above,  Dr.  Herschel  remarks  that  it  indicates 
a  high  degree  of  activity  in  regions  where,  but  for  such 
evidences,  we  might  conclude  all  lifeless. 

439.  "  I  am  disposed,"  says  Dr.  Dick,  "  to  consider  il 
as  highly  probable,  that  the  interposition  of  the  opake 
bodies  of  large  planets  revolving  around  such  stars  may, 
in.  some  cases,  account  for  the  phenomena. 

"  It  is  true  that  the  planets  connected  with  the  solar  system  are  so  small,  in  comparison 
of  the  sun,  that  their  interposition  between  that  orb  and  a  spectator,  at  an  immenee  dis- 
tance, would  produce  no  sensible  effect.  But  we  have  no  reason  to  conclude  that  in  all 
other  systems  the  planets  are  formed  in  the  same  proportions  to  their  central  orbs  as 
ours;  but  from  the  variety  we  perceive  in  every  part  of  nature,  both  in  heaven  and 
earth,  we  have  reason  to  conclude  that  every  system  of  the  universe  is,  in  some  re- 
spects, different  from  another.  There  is  no  improbability  in  admitting  that  the  planets 
which  revolve  round  some  of  the  stars  may  be  so  large  as  to  bear  a  considerable  propor- 
tion (perhaps  one-half  orons-third)  to  the  diameters  of  the  orbs  around  which  they  re- 
volve; in  which  case,  if  the  plane  of  their  orbit  lay  nearly  in  a  line  of  our  own  vision, 
they  would,  in  certain  parts  of  their  revolutions,  interpose  between  our  eye  and  the 
stars,  so  as  to  hide  for  a  time  a  portion  of  their  surfaces  from  our  view,  while  in  that 
part  of  their  orbits  which  is  next  to  the  earth." 

440.  Others,  again,  are  of  opinion  that  those  distant 
suns  have  one  luminous  and  one  opake  or  clouded  hemi- 
sphere ;  and  that  their  variations  may  thus  result  from  a 
revolution  upon  their  axes,  by  which  they  would  present 
us  alternately  with  their  full  and  their  diminished  luster. 

437.  What  other  specimen  ?    Usual  appearance?    Period?    Peculiar  color  ? 

438.  Cause  of  these  variations  ?     Supposition?     "Or.  IlersclicU 

439.  Dr.  Dick's  opinion  {     (Reasoning  in  note  ?) 

440.  What  other  hypothesis  stated? 


106  ASTRONOMY. 


Another  theory  is,  that  these  stars  are  moving  with 
inconceivable  velocity  in  an  immense  elliptical  orbit, 
the  longer  axis  of  which  is  nearly  in  a  direction  toward 
the  eye,  and  the  shorter  axis  of  which  would  be  imper- 
ceptible from,  our  system.  In  such  case,  the  star  would 
appear  alternately  to  approach  and  recede ;  now  looking 
in  upon  our  quarter  of  the  universe  for  a  few  days,  and 
then  rushing  back  into  immensity,  to  be  seen  no  more 
by  human  eyes  during  the  lapse  of  years  or  of  ages. 

441.  "  Whatever  may  be  the  cause"  says  Mr.  Abbott, 
"  the  fact  of  these  variations  is  perfectly  established,  and 
the  contemplation  of  the  stupendous  changes  which  must 
be  occurring  in  those  distant  orbs  overwhelms  the  mind 
with  amazement.     Worlds  vastly  larger  than  our  sun  sud- 
denly appear,  and  as  suddenly  disappear.     Now  they 
blaze  forth  with  most  resplendent  brilliancy,  and  again 
they  fade  away,  and  often  are  apparently  blotted  from 
existence.     These  worlds  are  unquestionably  thronged 
with  myriads  of  inhabitants  ;  and  the  phenomenon  which 
to  us  appears  but  as  the  waxing  or  waning  luster  of  a 
twinkling  star,  may,  to  the  dwellers  on  these  orbs,  be 
evolutions  of  grandeur,  such  as  no  earthly  imagination 
has  ever  conceived.     But  these  scenes,  now  veiled  from 
human  eyes,  will  doubtless  all  be  revealed,  when  the 
Christian  shall  ascend  on  an  angel's  wing  to  the  angel's 
home." 

TEMPORARY   STARS. 

442.  Temporary  stars  are  those  which  have  appeared 
from  time  to  time  in  different  parts  of  the  heavens. blaz- 
ing forth  with  extraordinary  luster,  and,  after  remaining 
for  a  while  apparently  immovable,  died  awray,  and  left 
no  traces  of  their  existence  behind.     Some  writers  class 
them  among  the  periodical  stars,  while  others  notice  them 
under  the  head  of  "  New  and  Lost  Stars." 

A  star  of  this  kind,  which  appeared  in  the  year  125 


441.  Remarks  of  Mr.  Abbott? 

442.  Whnt  are  temporary  stars?    How  classed?    First  noticed?    What. 
other  instance  ? 


TEMPORARY   STARS.  107 


B.  c.,  led  Hipparchus  to  draw  up  a  catalogue  of  the  stars 
the  earliest  on  record.  In  A.  D.  389,  a  similar  star  ap- 
peared near  the  largest  star  in  the  Eagle,  which,  after 
remaining  for  three  weeks  as  bright  as  Venus,  disap- 
peared entirely  from  view. 

443.  On  the  llth  of  November,  15Y2,  Tycho  Brahe,  a 
celebrated  Danish  astronomer,  was  returning,  in  the 
evening,  from  his  laboratory  to  his  dwelling-house,  when 
he  was  surprised  to  find  a  group  of  country  people  gazing 
upon  a  star  which  he  was  sure  did  not  exist  half  an  hour 
before.  It  was  then  as  bright  as  Sirius,  and  continued 
to  increase  till  it  surpassed  Jupiter  in  brightness,  and 
was  visible  at  noonday.  In  December  of  the  same  year 
it  began  to  diminish ;  and  in  March,  1574,  had  entirely 
disappeared. 

This  remarkable  star  was  in  the  constellation  Cassio- 
peia, about  5°  northeast  of  the  star  Caph.  The  place 
where  it  once  shone  is  now  a  dark  void ! 

4:4:4..  This  star  was  observed  for  about  16  months,  and 
during  the  time  of  its  visibility  its  color  exhibited  all  the 
different  shades  of  a  prodigious  flame.  "  First  it  was  of 
a  dazzling  white,  then  of  a  reddish  yellow,  and  lastly  of 
an  ashy  paleness,  in  which  its  light  expired."  "  It  is  im- 
possible," says  Mrs.  Somerville,  "  to  imagine  any  thing 
more  tremendous  than  a  conflagration  that  could  be  visi- 
ble at  such  a  distance." 

4:4:5.  In  reference  to  the  same  phenomenon,  Dr.  DICK 
observe.!,  that  "  the  splendor  concentrated  in  that  point 
of  the  heavens  where  the  star  appeared  must  have  been, 
in  reality,  more  than  equal  to  the  blaze  of  twelve  hundred 
thousand  worlds  such  as  ours,  were  they  all  collected 
into  one  mass,  and  all  at  once  wrapped  in  flames.  ISTay, 
it  is  not  improbable,  that  were  a  globe  as  large  as  would 
fill  the  whole  circumference  of  the  earth's  annual  orbit 
to  be  lighted  up  with  a  splendor  similar  to  that  of  the 


443.  What  other  remarkable  instance  described?     By  whom?    When 
in  what  constellation  ?     Position  ? 

444.  How  long  observed  ?    Appearance  ?    Mrs.  Sumervilie  ? 
•j-io.  Dr.  Dick's  remarks  ? 


198  ASTRONOMY. 


sun,  it  would  scarcely  surpass  in  brilliancy  and  splendor 
the  star  to  which  we  refer." 

446.  Kev.  Prof.  VINCE,  -who  has  been  characterized  as 
"  one  of  the  most  learned  and  pious  astronomers  of  the 
age,"  advances  the  opinion,  that  "  the  disappearance  of 
some  stars  may  be  the  destruction  of  that  system  at  the 
time  appointed  by  the  Deity  for  the  probation  of  its  in- 
habitants ;  and  the  appearance  of  new  stars  may  be  the 
formation  of  new  systems  for  new  races  of  beings  then 
called  into  existence  to  adorn  the  works  of  their  Creator." 

447.  LA  PLACE,  whose  opinion  upon  such  subjects  is 
always  entitled  to  consideration,  says  :  "  As  to  these  stars 
which  suddenly  shine  forth  with  a  very  vivid  light,  and 
then  immediately  disappear,  it   is    extremely  probable 
that   great   conflagrations,   produced   by   extraordinary 
causes,  take  place  on  their  surface.     This  conjecture  is 
confirmed  by  their  change  of  color,  which  is  analogous 
to  that  presented  to  us  on  the  earth  by  those  bodies  which 
are  set  on  fire,  and  then  gradually  extinguished." 

448.  Dr.  JOHN  MASON  GOOD,  author  of  the  Book  of 
Nature,  &c.,  seems  to  have  entertained  opinions  similar 
to  those  already  expressed.     u  Worlds  and  systems  of 
worlds,"  says  he,  u  are  not  only  perpetually  creating,  but 
also  perpetually  disappearing.     It  is  an  extraordinary 
fact,  that  within  the  period  of  the  last  century,  not  less 
than  thirteen  stars,  in  different  constellations,  seem  to 
have  totally  perished,  and  ten  new  ones  to  have  'oeen 
created.     In  many  instances,  it  is  unquestionable,  that 
the  stars  themselves,  the  supposed  habitations  of  other 
kinds  or  orders  of  intelligent  beings,  together  with  the 
different  planets  by  which  it  is  probable  they  were  sur- 
rounded, have  utterly  vanished,  and  the  spots  they  occu- 
pied in  the  heavens  have  become  blanks.     What  has 
befallen  other  systems  will  assuredly  befall  our  own.     Of 
the  time  and  manner  we  know  nothing,  but  the  fact  is 
incontrovertible;  it  is  foretold  by  revelation;   it  is  in- 
scribed in  the  heavens ;  it  is  felt  through  the  earth.    Such 
in  the  awful  and  daily  text.     What,  then,  ought  to  be  the 
comment  ?" 

440.  Prof.  Vince's  remarks ?     447.  La  Place's?     448.  Dr.  Goode'o* 


CLUSTERS   OF   STARS   AND   NEBULA.  1 99 


CHAPTER    IV. 

CLUSTERS     OF     STARS     AND     NEBULA. 

449.  IN  surveying  the  concave  of  the  heavens  in  a 
clear  night,  we  observe  here  and  there  groups  of  stars, 
forming  bright  patches,  as        TELESCOPIO  VIEW  OF  TIIK  PLKIADa, 
if  drawn  together  by  some 

cause  other  than  casual 
distribution.  Such  are  the 
Pleiades  and  Hyades  in 
Taurus.  These  are  called 
Clusters  of  Stars.  The 
luminous  spot  called  the 
Bee  Hive,  in  Cancer  (visi- 
ble to  the  naked  eye),  is 
somewhat  similar,  but  less 
definite,  and  requires  a 
moderate  telescope  to  re- 
solve it  into  stars.  In  the 
sword-handle  of  Perseus  is 
another  such  spot  or  clus- 
ter, which  is  also  visible  to 
the  naked  eye,  but  which 
requires  a  rather  better  telescope  to  resolve  it  into  dis- 
tinct Ktars.  When  fairly  in  view,  however,  it  is  one  of 
the  most  splendid  and  magnificent  spectacles  upon  which 
tli '5  oye  can  rest.  « 

"  O  what  a  confluence  of  ethereal  fires, 
From  worlds  unnumber'd  down  the  steep  of  heaven, 
Stream  to  a  point,  and  center  on  my  sight." 

450.  Many  of  these  faint  and  compact  clusters  have 
')een  mistaken  for  comets,  as  through  telescopes  of  mcd- 

449.  Clusters?     Specimens?  • 

4.50.  What  mistake  respecting?     What  like!     How  known  that  they  ara 
not  sx  -nets  ? 


200  ASTKUNOMT. 


BOUND  CLUSTER  IN  CAPKiror.N. 


erate  power  they  appear  like  such.  Messier  has  given  a 
list  of  103  objects  of  this  sort,  with  which  all  who  search 
for  comets  ought  to  be  familiar,  to  avoid  being  misled  by 
their  similarity  of  appearance.  That  they  are  not  comets, 
is  evident  from  their  fixedness  in  the  heavens,  and  from 
the  fact,  that  when  we  come  to  examine  them  with  in- 
struments of  great  power,  they  are  perceived  to  consist 
entirely  of  stars,  crowded  together  so  as  to  exhibit  a  deii 
nite  outline,  and  to  run  up  to  a  blaze  of  light  in  the  cen 
ter,  where  their  condensation  is  usually  the  greatest. 

451.  Some  of  these   clusters   are  of  an  exceedingly 
rough  figure,  and  convey  the  idea  of  a  globular  space 
filled  full  of  stars,  insulated  in  the  heaveos,  and  consti- 
tuting in  itself  a  family  or  s'o- 

ciety  apart  from  the  rest,  and 
subject  only  to  its  own  internal 
laws. 

It  would  be  a  vain  effort  to 
attempt  to  count  the  stars  in 
one  of  these  clusters.  They  are 
not  to  be  reckoned  by  hundreds  ; 
and  on  a  rough  calculation, 
grounded  on  the  apparent  inter- 
vals between  them  at  the  bor- 
ders, and  the  angular  diameter 

of  the  whole  group,  it  would  appear  that  many  clusters 
of  this  description  must  contain,  at  least,  from  ten  to 
twenty  thousand  stars,  compacted  and  wedged  together 
in  a  round  space,  whose  angular  diameter  does  not  ex- 
ceed eight  or  ten  minutes,  or  an  area  equal  to  a  tenth 
part  of  that  covered  by  the  moon. 

452.  Some  of  these  clusters  have  a  very  irregular  out- 
line.    These  are  generally  less  rich  in  stars,  and  especi- 
ally less  condensed  toward  the  center.     They  are  also  less 
definite  in  point  of  outline.     In  some  of  them,  the  stars 
are  nearly  all  of  a  size ;  in  others,  extremely  different. 
It  is  no  uncommon  thing  to  find  a  very  red  star,  much 

451.  What  said  of  the  form  of  these  clusters  ?    Stars  in  each  ?    Apparent 
diameter  ? 

452.  What  further  respecting  forms  ?    Character  of  irregular  clusters  ? 


NEBULJ3J.  201 


brighter  than  the  rest,  occupying  a  conspicuous  situation 
in  them. 

453.  It  is  by  no  means  improbable  that  the  individual 
stars  of  these  clusters  are  suns  like  our  own,  the  centers 
of  so  many  distinct  systems,  and  that  their  mutual  dis- 
tances   are   equal   to    those       EICH  CLXJSTEK  IN  BEKKNICEri,  HAIK. 
which  separate  our  sun  trom 

the  nearest  fixed  stars.  Be- 
sides, the  round  figure  ~of 
some  of  these  groups  seems 
to  indicate  the  existence  of 
some  general  bond  of  union, 
of  the  nature  of  an  attractive 
force. 

This  is  one  of  the  most  gorgeous  clusters 
in  all  the  heavens.  Sir  John  Herschel  pro- 
nounced it  the  most  magnificent  object  he 
had  ever  beheld.  It  is  about  6'  in  diameter, 
and  contains  a  countless  throng  of  stars, 
that  scarcely  ever  fail  to  elicit  a  burst  of  sur- 
prise and  astonishment  from  the  beholder! 
Who  can  gaze  upon  such  a  scene,  and  not  for  a  time  forget  earth,  In  the  rapt  contempla 
tion  of  the  distant  glory  ? 

"  There's  not  a  scene  to  mortals  given, 

That  more  divides  the  soul  and  clod, 
Than  yon  proud  heraldry  of  heaven — 
Yon  burning  blazonry  of  G6d." 

A  similar  cluster,  though  somewhat  different  in  form,  may  be  found  between  s  and  *7, 
in  Hercules.  This,  too,  is  a  most  magnificent  object.  Under  favorable  circumstances, 
it  may  be  seen  with  the  naked  eye ;  and  by  the  aid  of  telescopes,  it  is  easily  resolved 
into  myriads  of  stars.  "  It  is,  indeed,  truly  glorious,"  says  Smyth,  "and  enlarges  on  the 
eye  by  studious  gazing."  "  Perhaps,"  says  Prof.  Nichol,  •'  no  one  ever  saw  it,  for  the  first 
time,  through  a  telescope,  without  uttering  a  shout  of  wonder." 

NEBULJE. 

454.  The  term  Nebulas  is  applied  to  those  clusters  of 
stars  that  are  so  distant  as  to  appear  only  like  a  faint 
cloud  or  haze  of  light.     In  this  sense,  some  of  the  clus- 
ters heretofore  described  may  be  classed  as  nebulae  ;  and, 
indeed,  it  may  be  said  of  all  the  different  kinds  of  nebu- 
lae, that  it  is  impossible  to  say  where  one  species  ends, 
and  another  begins. 

453.  What  said  of  individual  stars  in  clusters  ?     Of  round  figure  of  some 
clusters?     (What  specimen  in  cut?     What  said  of  it  ?     Angular  diameter  ? 
Effect  of  seeing  ?     Poetry?     What  other  similar  cluster  ?     What  said  of  it '.} 

454.  What  are  Nebula  f    How  differ  from  clusters  ? 


202  ASTKONOMT. 


455.  Resolvable  Nebula  are  those  clusters,  the  light  o 
whose  individual  stars  are  blended  together,  when  seer, 
through  a  common  telescope ;  but  which,  when  viewed 
through  glasses  of  sufficient  power,  can  be  resolved  into 
distinct  stars. 

456.  Irresolvable   NelulcB  are   those   nebulous   spots 
which  were  formerly  supposed  to  consist  of  vast  fields  of 
matter  in  a  high  state  of  rarefaction,  and  not  of  distinct 
stars.     But  it  is  doubtful  whether  any  nebulae  exist  which 
could  not  be  resolved  into  stars,  had  we  telescopes  of 
sufficient  power. 

"  About  the  close  of  last  year,"  says  Dr.  Scoresby,  in 
1846,  uthe  Earl  of  Rosse  succeeded  in  getting  his  great 
telescope  into  complete  operation  ;  and  during  the  first 
month  of  his  observations  on  fifty  of  the  unresolvable 
nebulae,  he  succeeded  in  ascertaining  that  43  of  them 
were  already  resolvable  into  masses  of  stars.  Thus  is 
confirmed  the  opinion,  that  we  have  only  to  increase ^ the 
power  of  the  instrument  to  resolve  all  the  nebulae  into 
^tars,  and  the  grand  nebular  hypothesis  of  La  Place  into 

splendid  astronomical  dream." 

457.  Nebulae  of   almost    every 
conceivable  shape  may  be  found  in 
the   heavens.     Some   are   round — 
others  elliptical.     Some  occur  sin- 
gly, while   others   are   double,  or 
seem  to  be  connected  together. 

The  specimen  here  shown  is  in  the  Greyhound.  Tho 
two  nebulse  are  elliptical,  as  shown,  and  are  so  united 
os  to  stand  perpendicularly  to  each  other. 

458.  Annular  Nebulae,  are  those  that  exhibit  the  form 
of  a  ring.     Of  these,  but  few  specimens  are  known.     One 
of  the  most  striking  may  be  found  about  6°  below  Mizar, 

455.  What  are  resolvable  nebulae?    How  when  seen  through  powerful 
telescopes  ? 

456.  Irresolvable  nebulae?    Are  any  nebulae  really  irresolvable?    Remarks 
from  Dr.  Scoresby  ? 

457.  What  further  description  of  nebulse  ?    Specimen  ? 

458.  What  are  annular  nebula*  ?    Are  they  common  t    What  ppecimen  >i 
cut  ?    Describe  it. 


DOITBLS  NZMTIuS. 


NEBUL.E. 


203 
It 


ANNULAR   NEBULA. 


the  mid  ale  star  in  the  tail  of  the  Great  Bear. 
consists  of  a  large  and 
bright  globular  nebula, 
surrounded  by  a  double 
ring,  at  a  considerable 
distance  from  the  globe; 
Dr  rather  a  single  ring 
divided  through  about 
two-fifths  of  its  circum- 
ference, and  having  one 
portion  turned  up,  as  it 
were,  out  of  the  plane  of 
the  rest.  A  faint  .nebu- 
lous atmosphere,  and  a 
small  round  nebula  near 
it,  like  a  satellite,  com- 
pletes the  figure. 

459.  Another  very  conspicuous  nebula  of  this  class 
may  be  found  half-way  between  (3  and  7,  in  the  Lyre, 
and  may  be  seen  with  a  telescope  of  moderate  power. 
It  is  small,  and  particularly  well  defined,  so  as,  in  fact,  to 
have  much  more  the  appearance  of  a  flat  oval  solid  ring, 
than  of  a  nebula.     The  space  within  the  ring  is  filled 
with  a  faint  hazy  light,  uniformly  spread  over  it,  like  a 
fine  gauze  stretched  over  a  hoop. 

460.  " Planetary  Nelulce"  says  Dr.  Herschel,  "  are 
very  extraordinary  objects.     They  have,  as  their  name 
imports,  exactly  the  appearance  of  planets — round  or 
Slightly  oval  discs — in  some  instances  quite  sharply  ter- 
minated, in  others  a  little  hazy  at  the  borders,  and  of  a 
light  exactly  equable,  or  only  a  very  little  mottled,  which, 
in  some  of  them,  approaches  in  vividness  to  that  of  the 
actual  planets.     Whatever  be  their  nature,  they  must  be 
of  enormous  magnitude." 

461.  Stellar  Nebulce,  or  Nebulous  Stars,  are  such  as 
present  the  appearance  of  a  thin  cloud,  with  a  bright 
star  in  or  near  the  center.     They  are   round    or  oval- 

459.  What  other  annular  nebulae  1    Describe. 

4t50.  Planetary  nebulae  i    Describe. 

4(51.  /Stellar  nebulae  ?    Remarks  of  Professor  Mitchcl? 


204 


ASTRONOMY. 


STELLAR   NBBWUB. 


shaped,  and  look  like  a  star  with  a  burr  around  it,  or  a 
candle  shining  through  horn.  "It  was  an  object  of  this 
kind,"  says  Prof.  Mitchel,  "  which 
first  suggested  to  Sir  "W.  Ilerschel 
his  great  theory  of  the  formation 
of  suns  out  of  a  nebulous  fluid. 
He  thought  it  impossible  to  ac- 
count for  the  central  location  of 
stars,  surrounded  by  nebulous 
matter,  in  any  way  except  by  sup- 
posing this  to  be  a  sort  of  atmos- 
phere attracted  to,  and  sustained 
in  its  spherical  form  by,  the  power  of  the  central  body. 
I  have  examined  specimens  of  these  objects,  and  always 
with  increasing  wonder.  Their  magnitude  must  be  enor- 
mous, as  the  stars  are  certainly  not  nearer  than  other 
stars  ;  and  yet  the  circular  halo  around  them  is  of  a, 
diameter  easily  measured,  and  proves  them  to  have  a 
circumference  perhaps  greater  than  the  entire  orbit  of 
Neptune." 

462.  One  of 
the  most  remark- 
able nebula  in 
all  the  heavens 
may  be  found 
around  the  mid- 
dle star  in  the 
sword  of  Orion. 
It  is  easily  seen 
with  a  common 
telescope.  It  is 
shaped  like  the 
head  of  some 
animal — a  fish,  for  instance — with  its  mouth  open.  Near 
the  inner  surface  of  this  mouth  are  four  stars,  ranged  in 
the  form  of  a  trapezium.  It  requires  a  good  telescope 
to  see  four  stars ;  but,  with  powerful  instruments,  six  are 
visible,  instead  of  four. 


GREAT  NEBULA.   IN   OHIO 


462.  "Describe  the  nebula  of  Orion? 
stars  iu  5U 


Where  situated  ?     Shape  ?    What 


NEBULA.  205 


463.  The  sun  is  considered  by  astronomers  as  belong- 
ing to  this  class  of  nebulous  stars ;  and  the  Zodiacal 
Light  (322  and  325)  has  been  regarded  as  of  the  nature 
of  the  gaseous  matter  with  which  the  nebulous  stars  are 
surrounded.     It  is  supposed  that  if  we  were  as  far  from 
the  sun  as  from  the  stellar  nebulas,  he  would  appear  to  us 
only  as  a  small  and  nebulous  star ! 

464.  Until  recently,   the   most  powerful   instruments 
have  failed  to  reveal  any  thing  like  distinct  stars,  as  com- 
posing the  body  of  the   remarkable   nebula  in  Orion. 
Both  the  Herschels  regarded  it  as  positively  irresolvable ; 
or,  in  other  words,  as  composed  of  nebulous  fluid  or  un- 
organized matter.     But  it  has  recently  been  seen  to  be 
composed  of  distinct  stars,  both  by  the  monster  telescope 
of  Lord  Rosse,  and  the  great  refractor  of  Cambridge, 
near  Boston. 

465.  The  magnitude  of  this  nebula  must  be  beyond 
cill  human   conception.      "  If,"  says  Mr.  Smyth,   "  the 
parallax  of  this  nebula  be  no  greater  than  that  of  the 
stars,  its  breadth   cannot  be  less  than  a  hundred  times 
that  of  the  diameter  of  the  earth's  orbit ;  but  if,  as  is 
more  probable,  it  is  a  vast  distance  beyond  them,  its 
magnitude  must  be  utterly  inconceivable." 

466.  Prof.  Mitcliel  observes,  that  in  c£i£o  light  be  not 
absorbed  in  its  journey  through  the  celestial  spaces,  the 
light  of  the  nebula  of  Orion  cannot  reach  the  eye  in  less 
than  60,000  years,  with  a  velocity  of  twelve  millions  of 
miles  in  every  minute  of  time!     And  yet  this  object 
may  be  seen  from  this  stupendous  distance,  even  by  the 
naked  eye  !    What,  then,  must  be  its  dimensions  ?     Here, 
indeed,  we  behold  a  universe  of  itself  too  vast  for  the 
imagination  to  grasp,  and  yet  so  remote  as  to  appear  y 
taint  spot  upon  the  sky." 

467.  The  number  of  such  nebulous  bodies  is  unknown 


463.  Kemarks  respecting  the  sun  ? 

404.  How  the  nebula  in  Orion  regarded  ?    What  recent  discovery  ? 
465.  Its  probable  magnitude  ?    Kemark  of  Smyth  ? 

406.  Prof.  Mitchel's  observations  respecting  its  distance  and  dimensions? 
467.  What  said  of  the  number  of  nebulous  bodies  in  the  heavens  ?    Where 
n  ^st  abundant  I    II orschel's  catalogue  t    Various  forms  I 


206  ASTRONOMY. 


perhaps  we  should  say  innumerable.  The}7  are  especially 
abundant  in  the  Galaxy  or  Milky  Way.  Sir  W.  II  er- 
schel  arranged  a  catalogue,  showing  the  places  of  two 
thousand  of  these  objects.  They  are  of  all  shapes  and 
sizes,  and  of  all  degrees  of  brightness,  from  the  faintest 
milky  appearance  to  the  light  of  a  fixed  star. 

468.  Star  Dust  is  a  name  given  to  those  exceedingly 
faint  nebulous  patches  that  appear  to  be  scattered  about 
at  random  in  the  far-distant  heavens.     It  is  barely  visible 
through  the  best  telescopes,  and  seems  to  form  a  sort  of 
back-ground,  far  beyond  all  stars,  clusters,  and  nebulae, 
resolvable  or  irresolvable. 

469.  "  The.  nebulae,"  says  Sir  John  Ilerschel,  "  fur 
nish,  in  every  point  of  view,  an  inexhaustible  field  of 
speculation  and  conjecture.     That  by  far  a  larger  share 
of  them  consist  of  stars,  there  can  be  little  doubt ;  and 
in  the  interminable  range  of  system  upon  system,  and 
firmament   upon    firmament,    which   we   thus   catch   a 
glimpse  of,  the  imagination  is  bewildered  and  lost." 

470.  It  is  a  general  belief  among  astronomers  that  the 
material  universe  consists  of  distinct  clusters,  separated 
from  each  other  by  innumerable  chasms :  that  the  fixed 
stars  by  which  we  are  surrounded  constitute  one  great 
cluster — the  sun  being  a  star  with  the  rest,  arid  appearing 
as  he  does  to  us,  solely  on  account  of  our  nearness  to  him ; 
that  the  nebulae  are  far  beyond  our  cluster,  like  so  many 
distinct  continents  in  the  boundless  ocean  of  immensity. 

471.  Could  we  leave  our  system,  and  pass  outward 
toward  the  fixed  stars,  they  would  doubtless  expand  to 
the  dimensions  of  suns  as  we  approached  them,  while 
our  own  central  luminary  would  dwindle  to  a  glimmering 
star.     Reaching  the  frontier  of  the  cluster,  and  plunging 
off  into  the  awful  solitudes  of  space,  toward  the  distant 
nebulas  beyond,  we  should  see  them  also  expand  as  we 
drew  near,  while  our  vast  firmament  of  stars  seemed  to 

468.  What  is  meant  by  star  dust?    Where  supposed  to  be  situated  ? 
4t>9.  Herschel's  remark  respecting  the  nebulae  i 

470.  What  the  prevailing  opinion  among  astronomers,  as  to  the  structure 
of  the  universe  ? 

471.  What  imaginary  journey  and  scenery  described  by  the  author  ? 


NEBULA.  207 


be  gathering  into  a  compact  group ;  till  at  length,  enter- 
ing the  bosom  of  the  distant  nebulas,  we  should  find  our- 
selves surrounded  by  new  and  strange  constellations; 
and  if  we  saw  our  own  firmament  at  all,  should  see  it 
only  as  a  faint  annular  nebula,  far  beyond  the  reach  ot 
all  unassisted  vision. 

472.  The  great  stellar  cluster  in  which  the  sun  and 
solar  system  are  imbedded  is  supposed,  in  its  form,  to 
resemble  a  double  convex  lens,  with  the  sun  and  solar 
system  near  its  center;  and  by  being  viewed  edgewise 
from  our  central  position,  to  give  us  the  phenomenon  of 
the  Milky  Way. 

OHUAT  NKEULA  OF  Tlffl  8OLAE  SYSTEM. 


The  above  is  an  edgewise  view  of  the  great  stellar  cluster,  in  the  midst  of  which  tho 
solar  system  is  placed,  as  drawn  by  Sir  William  Herschel.  Its  figure  was  ascertained  by 
ganging  the  space-penetrating  power  of  his  telescope,  and  then  "  sounding  the  heavens," 
to  ascertain  the  distance  through  the  cluster,  in  all  directions,  to  the  open  void.  Tho 
nebula?  lie  in  distinct  and  independent  islands,  far  beyond  the  limits  of  our  cluster. 

Let  the  student  imagine  the  sun  to  be  one  of  the  stars  near  the  middle  of  the  lens- 
shaped  cluster,  of  which  the  above  is  an  edge  view,  with  the  planets  revolving  close 
around  it  If,  then,  he  look  out  upon  the  surrounding  stars,  the  number  visible,  and 
their  distinctness,  will  depend  upon  the  direction  in  which  he  looks.  If  toward  the 
thin  part  of  the  cluster  (either  up  or  down  in  the  cut),  fewer  stars  will  be  seen,  while 
they  will  be  comparatively  distinct  But  if  the  view  be  toward  the  edge  of  the  cluster, 
instead  of  the  sides  (or  horizontally,  in  the  cut),  there  will  be  seen  beyond  the  large 
stars,  and  fading  away  to  an  indistinct  and  mingled  light,  a  numberless  host  of  stars; 
and  this  zone  of  distant  stars  will  extend  quite  around  the  heavens.  Such  is  the  Galaxy 
or  Milky  Way.  The  zone  of  milky  light  is  the  light  of  the  stars  in  the  remote  edge  of 
the  great  cluster.  The  opening  in  the  left  end  of  the  figure  is  a  split  in  the  cluster,  and 
constitutes  the  division  seen  in  the  milky  way,  extending  part  way  around  the  heavens. 
See  cut  page  203. 

The  vast  apparent  extent  of  the  Galaxy,  as  compared  with  other  nebulje,  is  supposed 
to  be  justly  attributable  to  its  comparative  nearness.  Were  we  as  far  from  the  solar 
system  as  from  the  nebulae  in  the  Lyre,  the  Milky  Way  would  doubtless  appear  as  an 
annular  nebula  no  larger  than  that.  It  may  therefore  with  propriety  be  called  "tho 
great  nebula  of  the  solar  system." 

473.    Sir  W.   Herschel   estimated   that    50,000  stars 
passed  the  field  of  his  telescope,  in  the  Milky  Way v  in  a 

472.  Supposed  form  of  our  own  stellar  cluster  f    Philosophy  of  Galaxy 
(Why  apparently  PO  large  ?     How  appear  at  a  great  distance  ?) 

473.  Slurs  in  Milky  Way  ?    Mutual  distances  ?    Character  of  each  sfcvr? 


208  ASTRONOMY. 


single  hour!  And  yet  the  space  thus  examined  was 
hardly  a  point  in  the  mighty  concave  of  our  own  <;  sun 
strown  firmament."  What  an  idea  is  here  conveyed  to 
the  mind,  of  the  almost  boundless  extent  of  the  uni- 
verse !  The  mutual  distances  of  these  innumerable  orbs 
are  probably  not  less  than  the  distance  from  our  sun  to 
the  nearest  fixed  stars,  while  they  are  each  the  center  of 
a  distinct  system  of  worlds,  to  which  they  dispense  light 
and  heat. 

474.  "Were  the  universe  limited  to  the  Great  Solar 
Cluster,  in  the  midst  of  which  we  are  placed,  it  would 
be  impossible  to  conceive  of  its  almost  infinite  dimen- 
sions ;  but  when  we  reflect  that  this  vast  and  glowing 
zone  of  suns  is  but  one  of  thousands  of  such  assem- 
blages, which,  from  their  remoteness,  appear  only  as 
fleecy  clouds  hovering  over  the  frontiers  of  spa*ce,  we  are 
absolutely  overwhelmed  and  lost  in  the  mighty  abyss  of 
being ! 

475.  And  here  we  close  our  rapid  and  necessarily  im- 
perfect survey  of  the  Sidereal  Heavens.     And  while  the 
mind  of  the  student  is  filled  with  awe,  in  contemplating 
the  vastness  and  majesty  of  creation,  let  him  not  forget 
that  over  all  these  Jehovah  reigns — that  "  these  are  but 
parts  of  his  ways  ;"  and  yet  so  perfect  is  his  knowledge 
and  providence  in  every  world,  that  the  very  hairs  of 
our  heads  are  numbered,  and  not  a  sparrow  falls  without 
his  notice.     And  while  we  behold  the  wisdom,  power, 
and  goodness  of  God  so  gloriously  inscribed  in  the  heav- 
ens, let  us  learn  to  be  humble  and  obedient — to  love  and 
serve  our  Maker  here — that  we  may  be  prepared  for  the 
still  more  extended  scenes  of  another  life,  and  for  the 
society  of  the  wise  and  good  in  a  world  to  come. 

474.  Magnitude  of  our  own  cluster  ?    What  in  comparison  with  all  others? 

475.  Remarks  in  closing  paragraph  ?    Moral  reflections  ? 


PART  III, 

PRACTICAL   ASTRONOMY 


CHAPTER    I. 

PROPERTIES     OF     LIGHT. 

476.  Practical  Astronomy  has  respect  to  the  means 
employed  for  the  acquisition  of  astronomical  knowledge. 
It  includes  the  properties  of  light,  the  structure  and  use 
of  instruments,  and  the  processes  of  mathematical  calcu- 
lation. 

In  the  present  treatise,  nothing  farther  will  be  attempted  than  a  mere  introduction  to 
practical  astronomy.  In  a  work  designed  for  popular  use,  mathematical  demonstrations 
would  be  out  of  place.  Still,  every  student  in  astronomy  should  know  how  telescopes 
are  made  upon  what  laws  they  depend  for  their  power,  and  how  they  are  used.  It  ia 
for  this  purpose  mainly  that  we  add  the  following  chapters  on  Practical  Astronomy. 

477.  Light  is  that  invisible  ethereal  substance  by  whi'cli 
we  are  apprised  of  the  existence,  forms,  and  colors  of 
material  objects,  through  the  medium  of  the  visual  organs. 
To  this  subtile  fluid  we  are  especially  indebted  for  our 
knowledge  of  those  distant  worlds  that  are  the  principal 
subjects  of  astronomical  inquiry. 

478.  The  term  light  is  used  in  two  different  senses.   It 
may  signify  either  light  itself,  or  the  degree  of  light  by 
which  we  are  enabled  to  see  objects  distinctly.     In  this 
last  sense,  we  put  light  in  opposition  to  darkness.     But 

476.  Parts  of  the  book  gone  over  ?    Subject  of  Part  III.  ?    Of  Chapter  ].  I 
What  is  practical  astronomy?    (How  far  discussed  in  this  treatise  ?) 

477.  Define  light.    For  what  indebted  to  it  ? 

478.  Different  senses  in  which  the  term  is  used  ?    "What  is  darkness  ?    Can 
it  be  uark  and  light  at  the  same  time  ?    Is  there  any  place  without  light  ? 
(Quotation  from  Dick  ?) 


210  ASTRONOMY. 


it  should  be  borne  in  mind  that  darkness  is  merely  the 
absence  of  that  degree  of  light  which  is  necessary  to 
human  vision  ;  and  when  it  is  dark  to  us,  it  maybe  light 
to  many  of  the  lower  animals.  Indeed,  there  is  more  or 
less  light  even  in  the  darkest  night,  and  in  the  deepest 
dungeon. 

"  Those  unfortunate  individuals,"  says  Dr.  Dick,  "  who  have  been  confined  in  the  dark- 
est dungeons,  have  declared,  that  though,  on  their  first  entrance,  no  object  could  be  per- 
ceived, perhaps  for  a  day  or  two,  yet,  in  the  course  of  time,  as  the  pupils  of  their  eyes 
expanded,  they  could  readily  perceive  mice,  rats,  and  other  animals  that  infested  their 
cells,  and  likewise  the  walls  of  their  apartments;  which  shows  that,  even  in  such  situa- 
tions, light  is  present,  and  produces  a  certain  degree  of  influence," 

479.  Of  the  nature  of  the  substance  we  call  light  two 
theories  have  been  advanced.     The  first  is,  that  the  whole 
sphere  of  the  universe  is  filled  with  a  subtile  fluid,  which 
receives  from  luminous  bodies  an  agitation  j  so  that,  by 
its  continued  vibratory  motion,  we  are  enabled  to  per 
ceive  luminous   bodies.     This  was  the  opinion  of  Des- 
cartes, Euler,  Huygens,  and  Franklin. 

The  second  theory  is,  that  light  consists  of  particles 
thrown  off  from  luminous  bodies,  and  actually  proceeding 
through  space.  This  is  the  doctrine  of  Newton,  and  of 
the  British  philosophers  generally. 

"Without  attempting  to  decide,  in  this  place,  upon  the  relative  merits  of  these  t*v>  hy- 
potheses, we  shall  use  those  terms,  for  convenience  sake,  that  indicate  the  actual  passage 
of  light  from  one  body  to  another. 

480.  Light  proceeds  from  luminous  bodies  in  straight 
lines,  and  in  all  directions.     It  will  not  wind  its  way 
through  a  crooked  passage,  like  sound  ;  neither  is  it  con- 
lined  to  a  part  of  the  circumference  around  it. 

As  the  sun  may  be  seen  from  every  point  in  the  solar  system,  and  far  hence  into  space 
in  every  direction,  even  till  he  appears  but  a  faint  and  glimmering  star,  it  is  evident  that 
he  fills  every  part  of  this  vast  space  with  his  beams.  And  the  same  might  be  said  of 
every  star  in  the  firmament 

481.  As  vision  depends  not  upon  the  existence  of  light 
merely,  but  requires  a  certain  degree  of  light  to  emanate 
from  the  object,  and  to  enter  the  pupil  of  the  eye,  it  is 
obvious  that  if  we  can,  by  any  means,  concentrate  the 


479.  What  theories  of  the  nature  of  light,  and  by  whom  supported  respect- 
•  vely  ?     (Remark  of  author  ?) 

480.  How  light  proceeds  from  luminous  bodies  ?    (Eadiations  from  sun  and 
stars  ?) 

481.  How  improve  vision,  and  why  ?    (Animals  ?) 


REFRACTION   OF   LIGHT. 


211 


light,  so  that  more  may  enter  the  eye,  it  will  improve 
our  perception  of  visible  objects,  and  even  enable  us  to 
see  objects  otherwise  wholly  invisible. 

Some  animals  have  the  power  of  adapting  their  eyes  to  the  existing  degrw  of  light. 
The  cat,  horse,  &c.,  can  see  day  or  night;  while  the  owl,  that  sees  well  in  the  night,  sees 
poorly  in  the  day-time. 

482.  Light  may  be  turned  out  of  its  course  either  by 
reflection  or  refraction.  It  is  reflected  when  it  falls  upon 
the  highly  polished  surface  of  metals  and  other  intrans- 
parent  substances  ;  and  refracted  when  it  passes  through 
transparent  substances  of  different  densities. 


REFRACTION   OF   LIGHT. 

483.  Whenever  light  passes  from  a  rare  medium  to 
one  more  dense,  and  enters  the  latter  obliquely,  it  inva- 
riably leaves  its  first  direction,  and  assumes  a  new  one. 
This  change  or  bending  of  the  rays  of  light  is  what  is 

called  Refraction. 

The  term  refract  is  from  the  Latin  re,  and  frango,  to  break ;  and  signifies  the  break 
Ing  of  the  natural  course  of  the  rays. 

484.  As  air  and  water  are  both  transparent,  but  of 
different  densities,  it   follows   that,  when  light   passes 
obliquely  from  one  to 

the  other,  it  will  be 
refracted.  If  it  pass 
from  the  air  into  the 
water,  it  will  be  re- 
fracted toward  a  per- 
pendicular. 

Here  the  ray  A  C  strikes  the 
water  perpendicularly,  and  passes 
directly  through  to  B  without 
oeing  refracted.  But  the  ray  I)  C 
strikes  the  water  at  C  obliquely  ; 
and  instead  of  passing  straight 
through  to  E,  is  refracted  at  C. 
and  reaches  the  bottom  of  the 
water  at  F.  If,  therefore,  a  person  were  to  receive  the  ray  into  the  eye  at  F.  and  to 
judge  of  the  place  of  the  object  from  which  the  lijiht  emanates  from  the  direction  of  the 
ray  "C  F,  he  would  conclude  that  he  saw  the  object  at  G,  unless  he  made  allowance  for 
the  refraction  of  the  light  at  C. 


LIGHT  EEFKACTED  BY  WATEB. 
G  A 


E 


4S2.  How  \\fr\\t  turned  out  of  course  ? 

483.  \V  hat  is  refraction  ?    How  produced  ?    (Derivation  of  term  refract  'A 

184.  How  refracted  by  air  and  water?    (Illustrate  by  diagram.') 


212 


ASTRONOMY. 


LIGHT  PBOCEEDINO  FEOU  WA1EC. 

B 


485.  When    light 
passes    obliquely 
from  a  denser  to  a 
rarer     medium,    as 
from  water  into  air, 
it  is  refracted  from 
a  perpendicular  to- 
ward a  horizontal. 

Here  the  lamp  A  shines  up 
through  water  into  air.  The 
ray  that  strikes  the  surface  per- 
pendicularly passes  on  to  B 

without  being  refracted;   but  -0- 

the  other  rays  that  leave  the  water  obliquely  are  refracted  toward  a  horizontal  direction, 
in  proportion  to  their  distance  from  the  perpend icfular;  or,  in  other  words,  in  propor- 
tion to  the  obliquity  of  their  contact  with  the  surface  of  the  water. 

486.  In  consequence  of  the  refraction  of  light  toward 
a  horizontal  direction,  in  passing  from  water  into  air,  a 
pole,  half  of  which  is  in  the  water,  seems  bent  at  the 
surface,  and  the  lower  end  seems  nearer  the  surface  than 
it  really  is.      For    the 

same  reason,  the  bottom 
of  a  river  seems  higher, 
if  seen  obliquely,  than  it 
really  is  ;  and  the  water 
is  always  deeper  than 
we  judge  it  to  be. 

In  this  cut,  the  oar,  the  blade  of 
which  is  in  the  water,  seems  bent  at 
the  surface  of  the  water.  The  rays  of 
light  passing  from  the  part  under 
water  to  the  surface  at  D,  are  refract- 
ed toward  a  horizontal  direction  at 
that  point,  and  received  into  the  eye  of  the  observer  at  B,  who,  judging  of  the  position 
of  the  immersed  portion  of  the  oar  from  the  direction  of  the  rays  D  B,  locates  the  blado 
of  the  oar  at  C;  thus  reversing  the  effect  illustrated  at  484 

487.  The  refracting    power   of  different  transparent 
substances  depends  mainly  upon  their  density.     Water 
refracts  more  than  air,  glass  more  than  water,  and  dia- 
mond most  of  all.     But  the  angle  of  incidence,  or  the 
obliquity  of  the  contact  of  the  rays  with  the  denser  sub- 


EFFECT   OF   REFRACTION. 


485.  How  when  light  passes  from  denser  to  rarer  mediums  ?    (Diagram.) 

486.  Effect  of  refraction  upon  objects  seen  under  water  ?    (Diagram.) 

487.  Upon  what  does  the  refracting  power  of  different  transparent  media 
open<i  ? 


KEFKACTION   OF   LIGHT. 


213 


EFFECT  OF  KErBACTIOIT. 


stance,  has  also  much  to  do  in  determining  the  amount 
of  refraction. 

488.  By  the  aid  of  re- 
fraction, we  may  see  ob- 
jects that  are  actually  'be- 
hind an  opake  or  intrans- 
parent  body. 


Here  the  piece  of  money  at  A,  at  the 
bottom  of  the  cup,  would  be  invisible  to 
the  beholder  at  B,  if  the  cup  was  empty, 
its  the  light  from  the  money  would  pasa 
from  A'  to  C:  but  when  the  cup  is  filled 
with  water,  the  light  is  refracted  to  B, 
ami  the  beholder  sees  the  money  appa- 
rently at  D. 

489.  By  the  law  of  refraction,  light  has  been  found 
to  consist  of  a  combination  of  colors.  By  passing  a 
beam  of  light  through  a  triangular  piece  of  flint  glass 
called  a  prism,  it  is  seen  that  some  parts  of  the  light  are 
more  refrangible  than  others,  so  that  the  light  is  analyzed, 
or  separated  into  its  component  parts  or  elements. 


EEFRACTION  BY  A  PEIEM. 


Violet. 

Indigo 

Blue".. 

Green. 

Yellow 

Orange. 

Keel 

TVhite 

'  ^  Z. 

i^cta  ray  of  light  from  the  sun  be  admitted  throngh  a  hole  in  the  window  shntter,  A 
into  a  room  from  which  all  other  light  is  excluded  ;  it  will  form,  on  a  screen  placed  a  lit- 
tle distance  in  front,  a  circular  image,  B,  of  white  lisht.  Now,  interpose  near  the  shut- 
ter a  glass  prism.  C,  and  the  light,  in  passing  through  it,  will  not  only  be  refracted  in  the 
name  direction,  both  when  it  enters  the  prism  and  when  it  leaves  it,  but  the  several  rays 
of  which  white  light  is  composed  will  be  separated,  and  will  arrange  in  regular  order  on 
the  screen,  immediately  above  the  image  B,  which  will  disappear.  The  violet  ray,  it 
will  be  seen,  is  most  refracted,  and  the  red  least ;  the  whole  forming  on  the  screen  MI 
elongated  image  of  the  sun,  called  the  solar  spectrum. — Johnston. 


488.  What  other  effect  of  refraction  I    (How  illustrated  ?) 
4,89.  What  discovery  by  refraction  ?    (How  made  2) 


214 


ASTRONOMY. 


490.  It  is  the  refraction  of  the  clouds  that  ^ives  the 


sky  its  beautiful  colors 


and  evening ;  and  the 


refracting  power  of  the  rain-drops  produces  the  beautiful 
phenomenon  of  the  rainbow. 


ATMOSPHERICAL   REFRACTION. 

491.  The  refracting  power  of  the  atmosphere  producea 
many  curious  phenomena.     Sometimes  ships  are  seen 
bottom  upwards  in  the  air,  single  or  double.     At  other 
times,    objects   really  below   the   horizon,   as    ships   or 
islands,  seem  to  rise  up,  and  to  come  distinctly  in  view. 

492.  A  very  important  effect  of  refraction,  as  it  relates 
to  astronomy,  is,  that  it  more  or  less  affects  the  apparent 
places  of  all  the  heavenly  bodies.     As  the  light  coming 
from  them  strikes  the  atmosphere  obliquely,  and  passes 
downward  through  it,  it  is  refracted  or  bent  toward  the 
earth,  or  toward  a  perpendicular.     And  as  we  judge  of 
the  position  of  the  object  by  the  direction  of 'the  ray 
when  it  enters  the  eye,  we  place  objects  higher  in  the 
Leavens  than  they  really  are. 


ATMOSPHERICAL  REFRACTION. 


Let  A,  in  the  cnt,  represent  the  earth  ;  B,  the  atmosphere;  C  C,  the  visible  horizon  ; 
and  the  exterior  circle  the  apparent  concave  of  the  heavens.  Now,  as  the  light  passes 
from  the  stars,  and  strikes  the  atmosphere,  it  is  seen  to  curve  downward,  because  it 
strikes  the  atmosphere  obliquely ;  and  the  air  increases  in  density  as  we  approach  the 
earth.  But  as  the  amount  of  refraction  depends  not  only  upon  the  density,  but  also 
upon  the  obliquity  of  the  contact,  it  is  seen  that  the  refraction  is  greatest  at  the  horizon, 
and  gradually  diminishes  till  the  object  reaches  the  zenith,  when  there  is  no  obliquity,  and 
the  refraction  wholly  ceases.  The  dark  lines  in  the  cut  show  the  true,  and  the  dotted 
the  apparent  positions. 

490.  What  other  effects  of  refraction  ? 

4v»l.  Atmospherical  refraction?    Effects  on  terrestrial  objects ? 
4^2.  Upon  apparent  places  of  stars,  &c.  ?    (Diagram.    What  sjud  of  exo# 
gurataon  ?) 


ATMOSPHEEICAL   KEFR  ACTION.  215 


In  fbe  cut,  the  depth  of  the  atmosphere,  as  compared  with  the  globe,  is  greatly  exag- 
gerated. Even  allowing  it  to  be  50  miles  deep,  it  is  only  ft^th  of  the  semi-diameter  of 
the  globe,  which  is  equal  to  only  about  ^th  of  an  inch  upon  a  common  13-iuch  globe, 
rfut  it  was  necessary  to  exaggerate,  in  order  to  illustrate  the  principle. 

493.  The  amount  of  displacement  of.  objects  in  the 
horizon,  by  atmospherical  refraction,  is  about  33',  or  a 
little  more  than  the  greatest  apparent  diameter  of  eithei 
the  sun  or  moon.     It  follows,  therefore,  that  when  we 
bee  the  lower  edge  of  either  apparently  resting  on  the 
horizon,  its  whole  disk  is  in  reality  below  it ;  and  would  be 
entirely  concealed  by  the  convexity  of  the  earth,  were  it 
not  for  refraction. 

494.  Refraction  sometimes  causes  the  sun  and  moon 
to  appear  elongated  horizontally,  when  near  the  horizon, 
and  seen  through  a  dense  atmosphere.     The  rays  from 
their  lower  limb  being  refracted  more  than  those  from 
the  upper  limb,  on  account  of  coming  to  us  through  a 
lower  and  denser  portion  of  the  atmosphere,  the  lower 
portion  seems  higher  in  proportion  ;  or,  in  other  words, 
the   perpendicular  diameter   of  the    object    seems   the 
shortest.     It  is  then  called  a  horizontal  moon. 

495.  Another  effect  of  refraction  is,  that  the  sun  seems 
to  rise  about  three  minutes  earlier,  and  to  set  about  three 
minutes  later,   on  account  of  atmospherical  refraction, 
than  it  otherwise  would  ;  thus  adding  about  six  minutes, 
on  an  average,  to  the  length  of  each  day. 

The  atmosphere  is  said  to  be  so  dense  about  the  North  Pole  as  to  bring  the  sun  above 
the  horizon  some  days  before  he  should  appear,  according  to  calculation.  In  1596,  somo 
Dutch  navigators,  who  wintered  at  Nova  Zernbla,  in  latitude  76°,  found  that  the  sun  be- 
gan to  be  visible  17  days  before  it  should  have  appeared  by  calculation ;  and  Kepler 
computes  that  the  atmospheric  refraction  must  have  amounted  to  5°,  or  10  times  as 
much  as  with  us. 

496.  The  twilight  of  morning  and  evening  is  produced 
partly  by  refraction,  but  mainly  by  reflection.     In  the 
morning,  when  the  sun  arrives  within  18°  of  the  horizon, 
his  rays  pass  over  our  heads  into  the  higher  region  oi 
the  atmosphere,  and  are  thence  reflected  down  to  the 
earth.     The  day  is  then  said  to  dawn,  -and  the  light 
gradually  increases  till  sunrise.      In  the  evening,  this 

493.  Amount  of  displacement  of  celestial  objects  by  refraction  ?    Wbol 
follows  ? 

494.  What  effect  upon  apparent  form  of  moon,  &c.  ? 

495.  On  length  Of  days?     (How  about  North  Polo?) 
4'J6.  Cause  of  twilight?    (Note.) 


216 


ASTRONOMY. 


process  is  reversed,  and  the  twilight  lingers  till  the  snn 
is  183  below  the  horizon.  There  is  thus  more  than  an 
hour  of  twilight  both  morning  and  evening. 

In  (he  arctic  regions,  the  sun  is  never  more  than  18°  below  the  horizon;  so  that  th« 
twilight  continues  during  the  whole  night 

497.  In  making  astronomical  observations,  for  the  pur- 
poses of  navigation,  &c.,  allowance  has  to  be  made  for 
refraction,  according  to  the  altitude  of  the  object,  and 
the  state  of  the  atmosphere.  For  this  purpose  tables 
are  constructed,  showing  the  amount  of  refraction  for 
every  degree  of  altitude,  from  the  horizon  to  the  zenith. 


REFRACTION   BY   GLASS   LENSES. 

498.  A  lens  is  a  piece  of  glass  or  other  transparent 
substance,  of  such  a  form  as  to  collect  or  disperse  the 
rays  of  light  that  are  passed  through  it,  by  refracting 
them  out  of  a  direct  course.  They  are  of  different  forms, 
and  have  different  powers. 


In  the  adjoining  cut,  we  have  an  edgewise 
ix  different  " 


LENSES  OF  DIFFERENT  FORMS. 
BOD  K          t 


view  of  six  different  lenses. 

A  is  a  plano-convex,  or  half  a  double  con- 
vex lens ;  one  side  being  convex,  and  the  other 
plane. 

B  is  a  plano-concave  ;  one  surface  being  con- 
cave, and  the  other  plane. 

C  is  a  double-convex  lens,  or  one  that  is 
bounded  by  two  convex  surfaces. 

D  is  a  double-concave  lens,  or  a  circular 
piece  of  glass  hollowed  out  on  both  sides. 

E  is  a  concavo-convex  Ions,  whose  curves  differ,  but  do  not  meet,  if  produced. 

F  is  a  miniscus,  or  a  concavo-convex  lens,  the  curves  of  whose  surfaces  meet. 


499.  A  double-convex 
lens  converges  parallel 
rays  to  a  point  called 
the  focus  /  and  the  dis- 
tance of  the  focus  de- 
pends upon  the  degree 
of  convexity. 

In  the  first  of  these  cuts,  the  lens 
h  quite  thick,  and  the  focus  of  the 
rr.ys  is  quite  near;  but  the  other 
hoing  less  convex,  the  focus  is  more 
romoto. 


LIGHT  REFRACTED  BY  LENBES. 


497.  What  allowai.  Ji.  for  refraction  ?    Tables  ? 

498.  What  is  a  lens  ?    (Draw  and  describe  different  kinds  ?) 

499.  Refracting  powei  of  double-convex  lens?    Focal  distance?    (Diagram, 
<vid  illustrate.) 


ATMOSPHERICAL  REFRACTION. 


217 


.,„...         f—,,  , .     ,  /»  I>OTTBLE-CONVKX — FOCAL  DISTANCE. 

500.  The  distance  of 
the  focus  of  a  double-con- 
vex glass  lens  is  the  ra- 
dius of  the  sphere  of  its 
convexity. 

In  this  cut,  it  will  be  seen  that  the 
parallel  rays  A  are  refracted  to  a  focus 
nt  C,  by  the  double  convex  lens  B,  the 
convexity  of  whose  surfaces  is  juBt 
equal  to  the  curve  of  the  circle  D. 

501.  The  focal  distance  of  a  plano-convex  lens  is  equal 
to  the  diameter  of  the  sphere  formed  by  the  convex  surface 

produced.  PLANO-CONCAVE— FOCAL  DIOTANOB. 

It  must  be  borne  in  mind 
that  light  is  refracted  boUi 
when  it  enters  and  when  it 
leaves  a  double-convex  lens, 
end  in  both  instances  in  the 
same  direction ;  and,  so  far  as 
the  distance  of  the  focus  is  con- 
cerned, to  the  same  extent. 
But  when  the  lens  is  convex 
only  on  one  side,  half  its  re- 
fracting power  is  gone,  so  that 
the  rays  are  not  so  soon  re- 
fracted to  a  focus.  In  this  case, 
the  focal  distance  is  equal  to 
ttie  diameter  of  the  sphere 
formed  by  extending  the  convex  surface  of  the  lens ;  while  with  the  double-convex  lens, 
the  focal  distance  is  only  equal  to  the  radius  of  such -sphere.  In  the  cut,  the  parallel 
rays  A  are  refracted  to  a  focus  at  B,  by  the  plano-concave  lens  C ;  and  the  distance  C  B 
is  the  diameter  of  the  circle  D,  formed  by  the  convex  surface  of  the  lens  0  produced. 

502.  A  double- 
concave  lens   dis- 
perses parallel 
rays,   as    if   they 
diverged  from  the 
center  of  a  circle 
formed  by  the  con- 
vex   surface  pro- 
duced. 

In  this  cut,  the  parallel  rays  A  are  dispersed  by  the  double-concave  lens  B,  as  shown 
at  C;  and  their  direction,  as  thus  refracted,  is  the  same  as  if  they  proceeded  from  tno 
point  D,  which  is  the  center  of  a  circle  formed  by  the  concave  surface  of  the  lens  pi  in- 
duced. 


500.  How  focal  distance  governed  ?    (Diagram.) 
f»01.  What  is  the  focal  distance  of  &  plano-convex  lens  ?     (Diagram.) 
502.  Effect  of  douUct-wnwjc  lens  ?    Amount  of  divergency  of  rayu  ?    (Dia- 
gram.) 

10 


KAYS  DISPERSED   BY   EEFEACTIOM. 

0 


218 


ASTRONOMY. 


BUENINQ-GLAbS, 


503.  Common  spectacles,  opera-glasses,  burning-glasses, 
and  refracting  telescopes  are  made  by  converging  light 
to  a  focus,  by  the  use  of  double-convex  lenses. 

The  ordinary  burning-glass,  which  may  be  bought  for  a 
few  shillings,  is  a  double-convex  disk  of  glass  two  or  three 
inches  in  diameter,  inclosed  in  a  slight  metallic  frame 
with  a  handle  on  one  side. 
Old  tobacco-smokers  some- 
times carry  them  in  their 
pockets,  to  light  their  pipes 
with  when  the  sun  shines. 
In  other  instances,  they  have 
been  so  placed  as  to  fire  a 
cannon  in  clear  weather,  by 
igniting  the  priming  at  12 
o'clock. 

The  adjoining  cut  represents  a  large  bnrn- 
Ing-glass  converging  the  rays  of  the  sun  to  a 
focus,  and  setting  combustible  substances  on 
fire.     Such  glasses  have  been  made  power- 
ful enough  to  melt  the  most  refractory  sub- 
stances, as  platinum,  agate,  &c.    "  A  lens  three  feet  in  diameter,"  says  Professor  Grny, 
u  has  been  known,  to  melt  carnelian  in  75  seconds,  and  a  piece  of  white  agate  in  SO 
seconds." 


REFLECTION   OF   LIGHT. 

504.  We  have  now  shown  how  light  may  be  turned 
out  of  its  course,  and  analyzed,  dispersed,  or  converged 
to  a  point  by  refraction.     Let  us  now  consider  how  it 
may  be  converged  to  a  focus  by  reflection. 

505.  When  light  falls  upon  a  highly  polished  surface, 
especially  of  metals,  it  is  reflected  or  thrown  off  in  a  new 
direction,  and  the  angles  of  con- 

tact  and  departure  are  always 
equal. 

Let  A  B  represent  the  polished  metallic  sur- 
face. C  the  source  of  light,  and  the  arrows  the 
direction  of  the  ray.  Then  D  would  represent 
lue  angle  of  incidence  or  contact  and  E  the  angle 
of  reflection  or  departure  —  which  angles  are  seen 
to  be  equal. 


MFLKCTIolf  BY  A 


503.  What  articles  made  with  double-ec  nvex  lenses?    U»es  ?    (Power  01 
burning-glasses  ?) 

504.  What  now  shown  in  this  chapter  ?    What  next  ? 

505.  What  is  rwCfetiMfc  and  when  does  it  take  piaee  ?    What  law  govern* 
it?    (Diagram.) 


REFLECTION   OF   LIGHT. 


506.  A  concave  mirror  reflects  parallel  rays  back  to  a 
focus,  the  distance  of  which  is  equal  to  half  the  radius 
of  the  sphere  formed  by  the  concave  surface  produced. 


BEFLECTION  BT  A  CONCAVE  MIKKOR. 


In  this  cut,  the  parallel  rays  A  fall  upon  the  concave  mirror  B  B,  and  are  reflected  to 
the  focus  C,  which  is  half  the  radius  of  the  sphere  formed  by  the  surface  of  the  mirror 
produced.  If,  therefore,  it  was  desirable  to  construct  a  concave  mirror,  having  its  focus 
10  feet  distant,  it  would  only  be  necessary  to  grind  it  on  the  circle  of  a  sphere  having  a 
nidi  us  of  20  feet.  ~. 

507.  In  reflection,  a.  portion  of  the  light  is  absorbed  or 
otherwise  lost,  so  that  a  reflector  of  a  given  diameter  will 
not  converge  as  much  light  to  a  focus  as  a  double-convex 
lens  of  the  same  size.  In  the  latter  case,  all  the  light  is 
transmitted.  Still,  reflectors  have  been  formed  of  such 
power  as  to  melt  iron,  and  other  more  difficult  sub- 
stances. 

We  have  now  considered  so  much  of  optics  as  is  necessary  to  an  understanding  of  the 
principles  upon  which  telescopes  are  constructed;  and,  for  further  particulars,  shall  refer 
tl'.e  student  to  books  of  Natural  Philosophy. 

506.  How  does  a  concave  mirror  reflect  parallel  rays  ?    Distance  of  focus  ? 
(Diagram.     How  would  you  construct  a  concave  mirror  with  a  10  feet  focus  ?) 

507.  Is  all  the  light  falling  upon  a  polished  surface  reflected  ?    What  then  f 
(Closing  note  ?) 


£20  ASTRONOMY. 


CHAPTER     II. 

TELESCOPES. 

508.  A  Telescope  is  an  optical  instrument  employed  in 
viewing  distant  objects,  especially  the  heavenly  bodies. 
The  term  telescope  is  derived  from  two  Greek  words,  viz., 
tele,  at  a  distance,  and  skopeo,  to  see. 

509.  So  far  as  is  now  known,  the  ancients  had  no 
knowledge  of  the  telescope.     Its  invention,  which  oc- 
curred in  1609,  is  usually  attributed  to  Galileo,  a  phi- 
losopher of  Florence,  in  Italy. 

The  discovery  of  the  principle  upon  which  the  refracting  telescope  is  constructed 
was  purely  accidental.  The  children  of  one  Jansen,  a  spectacle-maker  of  Middleburgh, 
in  Holland,  being  at  play  in  their  father's  shop,  happened  to  place  two  glasses  in  such  a 
manner,  that  in  looking  through  them,  at  the  \veather-cock  of  the  church,  it  appeared 
to  be  nearer  and  much  larger  than  usual.  This  led  their  father  to  fix  the  glasses  upon  a 
board,  that  they  might  be  ready  for  observation ;  and  the  news  of  the  discovery  was  soon 
conveyed  to  the  learned  throughout  Europe.  Galileo  hearing  of  the  phenomenon,  soon 
discovered  the  secret,  and  put  the  glasses  in  a  tube,  instead  of  on  a  board ;  and  thus  the 
first  telescope  was  constructed. 

510.  The  telescope  of  Galileo  was  but  one  inch  in  di 
ameter,  and  magnified  objects  but  30  times.     Yet  with 
this  simple  instrument  he  discovered  the  face  of  the 
moon  to  be  full  of  inequalities,  like  mountains  and  val- 
leys; the  spots  on  the  sun  ;  the  phases  of  Venus  ;  the  satel- 
lites of  Jupiter ;  and  thousands  of  new  stars  in  all  parts 
of  the  heavens. 

Notwithstanding  this  propitious  commencement,  so  slow  was  the  progress  of  the 
telescope  toward  its  present  state,  that  in  1S16,  Bonnycastle  speaks  of  the  80-fold  mag- 
nifying power  of  the  telescope  of  Galileo  as  "nearly  the  greatest  perfection  that  this 
kind  of  telescope  is  capable  of  I" 

511.  If  he  be  the  real  author  of  an  invention  who, 
from  a  knowledge  of  the  cause  upon  which  it  depends, 
deduces  it  from  one  principle  to  another,  till  he  arrives 

508.  Subject  of  Chap.  II.  ?    Telescope  ?    Derivation  ? 

509.  Ancient  or  modern  ?    Inventor  ?    (Incidents  of  discovery  ?) 

510.  Galileo's  telescope?     Discoveries  with  it?     (Progress  in  telescoro 
making  ?) 

511.  IB  Galileo  entitled  to  the  lionor  of  i-  venting  the  telescope?    (AA  tsro 
'a  his  ?) 


DIFFERENT   KINDS   OF   TELESCOPES.  221 


at  the  end  propobed,  then  the  whole  merit  of  the  inven- 
tion of  the  telescope  belongs  to  Galileo.  The  telescope 
of  Jansen  was  a  rude  instrument  of  mere  curiosity,  acci- 
dentally arranged ;  but  Galileo  was  the  first  who  con- 
structed it  upon  principles  of  science,  and  showed  the 
practical  uses  to  which  it  might  be  applied. 

It  is  said  that  the  original  telescope  constructed  by  Galileo  Is  still  preserved  in  the 
Britihh  Museum.  A  pigmy,  indeed,  in  its  way,  but  the  honored  progenitor  of  a  race  of 
giants! 

512.  The  discovery  of  the  telescope  tended  greatly  to 
sustain  the  Copernican  theory,  which  had  just  been  pro- 
mulgated (10),  and  of  which  Galileo  was  an  ardent  dis- 
ciple. Like  Copernicus,  however,  his  doctrines  subjected 
him  to  severe  persecutions,  and  he  was  obliged  to  re- 
nounce them. 

The  following  is  his  renunciation,  made  June  28, 1633 :  "  I,  Galileo,  in  the  seventieth 
year  of  my  age,  on  bended  knees  before  your  eminences,  having  before  my  eyes  and 
touching  with  my  hands  the  Holy  Gospels,  I  curse  and  detest  the  error  of  the  earth's 
movement"  As  he  left  the  court,  however,  after  this  forced  renunciation,  he  is  said 
to  have  stamped  upon  the  earth,  and  exclaimed,  "  It  does  move,  after  all !"  Ten  years 
nt'ter  this  he  was  sent  to  prison  for  the  same  supposed  error;  and  soon,  his  age  advan- 
cing, the  grave  received  him  from  the  malice  of  his  persecutors. 


DIFFERENT   KINDS   OF   TELESCOPES. 

513.  Telescopes  are  of  two  kinds — Reflectors  and  Re- 
fractors.   Refracting  telescopes  are  made  by  refracting 
the  light  to  a  focus  with  a  glass  lens  (499) ;  and  reflect- 
ing telescopes,  by  reflecting  it  to  a  focus  with  a  concave 
mirror  (506).     Besides  this  general  division,  there  are 
various  kinds,  both  of  reflectors  and  refractors. 

514.  Telescopes  assist  vision  in  various  ways — first, 
by  enlarging  the  visual  angle  under  which  a  distant  ob- 
ject is   seen,  and  thus  magnifying   that   object ;    and, 
secondly,  by  converging  to  a  point  more  light  than  could 
otherwise  enter  the  eye — thus  rendering  objects  distinct 
or  visible  that  would  otherwise  be  indistinct  or  invisible. 

All  the  light  falling  upon  a  six  or  a  twelve  inch  lens  may  be  converged  to  a  focus,  so 
as  to  be  taken  into  the  human  eye  through  the  pupil,  which  is  but  one-fourth  of  an  inch 
in  diameter.  Our  vision  is  thus  made  as  perfect  by  art  as  if  nature  had  given  us  ability 
to  enlarge  the  eye  till  the  pupil  was  a  foot  in  diameter. 

512.  Eelation  of  discovery  to  Copernican  theory  ?    Effects  upon  Galileo  ? 
(His  renunciation  ?    Death  ?.) 

513.  Kinds  of  telescopes  ?    Describe. 

514.  How  os.-ust  vision  ?    (Illustrative  note  ?) 


£22 


ASTEONCXMY. 


515.  Refracting  telescopes  may  consist  of  a  double- 
couvex  lens  placed  upon  a  stand,  without  tube  or  eye- 

Eiece.     Indeed,  a  pair  of  ordinary  spectacles  is  nothing 
jss  than  a  pair  of  small  telescopes,  for  aiding  impaired 


vision. 


REFRACTING  TELESCOPE  WITH  A  SINGLE  LEXS. 


Here  the  parallel  rays  ar«  seen  to  pass  through  the  lens  at  A,  and  to  be  so  converged  to 
a  point  as  to  enter  the  eye  of  the  beholder  at  B.  His  eye  is  thus  virtually  enlarged  to  the 
size  of  the  lens  at  A.  But  it  would  be  very  difficult  to  direct  such  a  telescope  toward 
celestial  objects,  or  to  get  the  eye  in  the  focus  after  it  was  thus  directed. 

516.  The  Galilean  telescope  consists  of  two  glasses — 
a  double-convex  next  the  object,  and  a  double-concave 
near  the  eye.  The  former  converges  the  light  till  it  can 
be  received  by  a  small  double-concave,  by  which  the  con- 
vergency  is  corrected  (502),  and  the  rays  rendered  paral- 
lel again,  though  in  so  small  a  beam  as  to  be  capable  of 
entering  the  eye. 


GALILEAN   TELESCOPE. 


Here  the  light  is  converged  by  the  lens  A,  till  it  can  be  received  by  the  double-con- 
cave lens  B,  by  which  the  rays  are  made  to  become  a  small  parallel  beam,  that  can  enter 
the  eye  at  C.  This  was  the  form  of  the  telescope  constructed  by  Jansen,  and  improved 
by  Galileo ;  on  which  account  it  is  called  the  Galilean  telescope.  In  the  cut,  the  two 
lenses  are  represented  as  fastened  to  a  board,  as  first  exhibited  by  Jansen. 

517.  The  common  astronomical  telescope  consists  of 
two  glasses — viz.,  a  large  double-convex  lens  next  tho 


15.  Simplest  form  of  refracting  telescope  ?    (Diagram  ?) 

16.  Galilean  telescope  I    (Diagram  and  < 


515. 
516. 
lean?) 
617.  How  common  astronomical  telescopes  made  ?    Why  in  tube  t 


explanation  I    Wiry  called  Gall- 


DIFFERENT   KINDS   OF  TELESCOPES. 


223 


object,  called  the  object-glass  /  and  a  small  double-convex 
lens  or  microscope  next  the  eye,  called  the  eye-piece.  For 
the  greater  convenience  in  using,  they  are  both  placed  in 
a  tube  of  wood  or  metal,  and  mounted  in  various  ways, 
according  to  their  size,  and  the  purposes  to  which  they 
are  devoted. 


LKXSES  PLACKD  IN  A  TUBE. 


REFRACTING   TELESCOPE  MOUNTED   ON  A  STAND. 


A  is  the  object-glass,  B  the  eye-piece,  and  C  the  place  where  the  tube  in  which  the 
eye-piece  is  set  slides  in  and  out  of  the  large  tube,  to  adjust  the  eye-piece  to  the  focal 
ilistance.  By  placing  the  lenses  in  a  tube,  the  eye  is  easily  placed  in  the  focus,  and  th* 
object-glass  directed  toward  any  desired  object. 

518.  The  object-glass  of  a  telescope  is  usually  pro- 
tected, when  not  in  use,  by  a  brass  cap  that  shuts  over 
the  end  of  the  instrument ;  and  the  eye-pieces,  of  which 
there  are  several,  of  different  magnifying  powers,  are 
so  fixed  as  to  screw 
into  a  small  movable 
tube  in  the  lower  end 
of  the  instrument,  so 
as  to  adjust  them  re- 
s -Actively  to  the  fo- 
cus, and  to  the  eyes 
of  different  observ- 
ers. Such  telescopes 
usually  represent  ob- 
jects in  an  inverted 
position. 

The  adjoining  cut  represents 
the  simplest  form  of  a  mounted 
refractor.  The  object-glass  is  at 
A,  where  the  brass  cap  may  be 
seen  covering  it.  B  is  the  small 
tube  into  which  the  eye-piece 
is  screwed,  and  which  is  moved 
in  and  out  by  the  small  screw 
C.  Two  eye-pieces  muy  be  seen 
at  D — one  short  one,  fur  astro- 
nomical observations;  and  a 
long  one,  for  hind  objects.  For 

-  iewing  the  sun,  it  is  necessary  to  add  a  screen,  made  of  colored  glass.     At  E,  a  bolt  goes 
i  ito  a  socket  in  the  top  of  the  stand,  in  which  it  turns,  allowing  the  telescope  to  sweep 

518.  How  object-glass  protected?  What  suid  of  eye-pieces?  (Cut  and 
explanation  ?) 


224  ASTRONOMY. 


erpund  the  horizon;  while  the  joint,  connecting  the  saddle  in  which  the  telescope  rests 
with  the  top  of  the  bolt,  allows  it  to  be  directed  to  any  point  between  the  horizon  and 
the  zenith.  But  such  stands  answer  only  for  comparatively  small  instruments. 

510.  Refracting  telescopes  are  mounted  in  various 
ways.  So  important  is  it  that  they  should  not  shake  01 
vibrate,  that,  in  most  observatories,  the  stand  rests  upon 
heavy  mason-work  in  no  way  connected  with  the  build- 
ing, so  that  neither  the  wind  nor  the  tread  of  the  ob- 
server can  shake  it.  They  are  then  furnished  with  a 
double  axis,  which  allows  of  motion  up  and  down,  or 
east  and  west ;  and  two  graduated  circles  show  the  pre- 
cise amount  of  declination  and  right  ascension.  They 
are  then  furnished  with  clockwork,  by  which  the  tele- 
scope is  made  to  move  westward  just  as  fast  as  the  earth 
turns  eastward ;  so  that  the  celestial  object  being  once 
found,  by  setting  the  instrument  for  its  right  ascension 
and  declination,  or  by  the  aid  of  the  Finder — a  small 
telescope  attached  to  the  lower  end  of  the  large  one — it 
may  be  kept  in  view  by  the  clockwork  for  any  desirable 
length  of  time.  A  telescope  thus  furnished  with  right 
ascension  and  declination  circles  is  called  an  Equatorial, 
or  is  said  to  be  eqnatwially  'mounted,  because  it  sweeps 
east  and  west  in  the  heavens  parallel  to  the  equator. 

520.  The  object-glasses  of  telescopes  are  not  always 
made  of  a  single  piece  of  glass.     They  may  be  made  of 
two  concavo-convex  glasses,  like  two  watch  crystals,  with 
their  concave  sides  toward  each  other,  or  with  a  thin 
double  concave  glass  between  them.     They  are  thus 
double,  or  triple  ;  but  when  thus  constructed,  the  whole 
is  called  a  lens,  as  if  composed  of  a  single  piece.     Lenses 
have  also  been  formed  by  putting  two  concavo-convex 
glasses  together,  and  filling  the  space  between  them  with 
some  transparent  fluid.     These  are  called  Barlow  lenses, 
from  Prof.  Barlow,  their  inventor. 

521.  As  a  prism  analyzes  the  light,  and  exhibits  dif- 
ferent colors,  so  a  double-convex  lens  may  analyze  the 


519.  How  refractors  mounted,  and  why  ?    When  equatorial,  and  why  ? 

520.  How  object-glasses  made  ?    What  a  lens  ?    A  Barlow  lens  ( 

521.  What  is  an  Achromatic  telescope  ?    ( Derivation  of  term  ?) 


DIFFERENT   KINDS   OF   TELESCOPES. 

light  that  falls  near  its  circumference,  and  thus  represent 
the  outside  of  the  heavenly  bodies  as  colored.  But  this 
defect  is  remedied  by  using  disks  made  of  different  kinds 
of  glass,  so  as  to  correct  one  refraction  by  another.  Re- 
fracting telescopes  thus  corrected  are  called  Achromatic 
telescopes. 

Achromatic  is  from  the  Greek  a  chroma,  which  signifies  destitute  of  color.  Most 
refracting  telescopes  are  now  so  constructed  as  to  be  achromatic. 

522.  It  is  but  recently  that  any  good  refracting  tele- 
scopes have  been  made  in  this  country.  The  best  have 
been  made  in  Germany  and  France.  Several  very  good 
instruments  have  been  made  by  Alvan  Clark,  Esq.,  of 
Boston,  Charles  A.  Spencer,  Esq.,  of  Troy,  N.  Y.,  and 
the  late  Henry  Eitz,  Jun.,  of  New  York  City. 

The  author  was  personally  well  acquainted  with  Mr.  Fitz,  and  during  his  life  gave 
favorable  descriptions  of  his  instruments  in  tbase  pages,  and  did  all  that  he  could  to 
make  his  capabilities  known  to  the  American  public.  He  made  his  first  telescope  in 
Io85.  In  the  winter  of  1841  he  invented  a  method  of  perfecting  object-glasses  for 
refracting  telescopes,  making  the  first  one  of  the  bottom  of  an  ordinary  tumbler,  la 
the  fall  of  1845  he  exhibited  at  the  Fair  of  the  American  Institute,  an  instrument  of  6 
inches  aperture,  which,  although  made  of  common  American  material,  in  the  way  of 
flint  glass,  was  a  very  excellent  instrument.  It  secured  him  the  friendship  of  noted 
astronomers,  and  from  that  time  forward  he  devoted  himself  to  the  business  of 
telescope-making  with  a  good  degree  of  success.  Continually  progressing  in  size,  he 
finally  succeeded  in  making  instruments  of  16  inches  aperture,  one  of  which  is  now 
in  the  possession  of  Mr.  Van  Dusee,  of  Buffalo,  N.  \.  lie  made  two  of  18  inches,  ono 
for  the  Dudley  Observatory,  at  Albany,  and  the  other  for  an  association  of  gentlemen 
at  Allegheny  City,  Pa.  Of  12  inches  aperture  he' produced  one  for  the  Observatory  at 
Ann  Arbor,  Michigan,  and  another  for  the  Vassar  Female  College.  He  made  for  Mr. 
L.  M.  Rutherford,  of  New  York,  at  various  times,  telescopes  of  4,  5J,  6,  9,  and  11J 
inches  aperture;  the  last,  an  Instrument  of  remarkable  defining  power,  is  now 
mounted  in  Mr.  Rutherford's  Observatory  in  Eleventh  Street,  New  York  city.  Mr. 
Vickers  of  Baltimore  has  a  10-inch.  Several  of  the  size  of  8  and  9  inches  are  scattered 
over  the  country.  The  British  Charge^  <T Affaires  at  Montevideo  has  a  9-inch.  Mr. 
Campbell,  of  New  York,  has  an  8-iuch.  Of  a  large  number  of  6  inches  aperture,  ono 
very  fine  instrument  was  ordered  by  the  United  States  Government  for  Lieut.  Gillies's 
expedition  to  Chili;  it  is  still  in  the  Observatory  of  the  Chilian  Government.  At 
about  the  same  time  he  made  another  of  the  same  size  for  Mr.  Robert  Van  Arsdale, 
of  Newark,  N.  J  Mr.  Thomas  P.  Harrison,  Principal  of  the  Public  Grammar 
School  in  Greenwich  Avenue,  New  York,  has  another  mounted  on  that  building.  In 
1869,  M.  Foucault  announced  to  the  French  Academy  of  Science  a  great  discovery  in 
the  manufacture  of  telescopes — local  polishing — a  discovery  that  Mr.  Fitz  had  been 
using  for  fifteen  years.  When  he  was  seized  with  his  last  illness  he  was  about  to  go  to 
Europe  to  select  a  glass  for  a  24-inch  telescope,  the  ambition  of  his  later  years. 

522.  What  eald  of  the  manufacture  of  telescopes  ?  What  Americana 
bave  made  them  ?  What  said  of  Mr.  Fitz's  telescopes  ? 

10* 


£26 


ASTRONOMY. 


EUTHEEFOKD'S  EQUATORIAL  KEFKACTOK. 

523.  The  above  cut  represents  an  equatorial  telescope 
manufactured  by  Mr.  Henry  Fitz,  of  New  York — the  one 
used  by  the  author  in  making  most  of  his  observations. 
Its  object-glass  is  six  inches  in  diameter,  and  its  focal 
length  eight  feet. 

A  is  the  declination  circle,  and  B  the  circle  of  right  ascension.  The  two  sticks  hang- 
Ing  from  these  circles  are  used  to  move  the  instrument  in  right  ascension  or  declination, 
while  the  observer  is  at  the  eye-piece. 

The  Finder  is  seen  attached  to  the  lower  end  of  the  large  instrument  It  takes  in 
a  larger  field  of  view  in  the  heavens  than  the  latter,  and  enables  the  observer  to  look 
up  objects  with  facility,  and  bring  them  into  the  field  of  the  larger  instrument 

This  instrument  has  no  clockwork  attached.  It  rests  upon  a  pillar  of  heavy  mason- 
work,  the  top  of  which  may  be  seen  in  the  cut ;  and  in  the  hands  of  its  present  owner, 
Lewis  M.  Rutherford,  Esq.,  has  already  rendered  very  eflMent  service. 

5:23.  Mr.  Rutherford's  telescope?    By  whom  made? 


DIFFERENT  KINDS   OF  TELESCOPES. 


007 
*r£fl 


GIIEAT   BBFRACTING  TELESCOPE   AT   CINCINNATI,    OHIO. 

524:.  The  above  cut  represents  one  of  the  most  im- 
portant telescopes  in  the  United  States.  It  is  located 
in  the  observatory  on  Mount  Adams,  near  Cincinnati, 
Ohio,  and  was  for  several  years  under  the  direction  of 
the  late  Prof.  O.  M.  Mitchel,  by  whose  instrumentality 
it  was  purchased  and  mounted. 

The  object-glass  is  aboii1. 12  inches  in  diameter,  with  a  focal  distance  of  17  feet  It 
HTns  purchased  in  Munich,  Germany,  in  1S44,  at  an  expense  of  nearly  ten  thousand 
dollars.  There  is  now  but  one  better  one  in  this  country,  though  probably  several 
larger  instruments  than  this  is. 

524.  Cincinnati  refractor — where  located?  By  whom  purchased? 
(Where?  When?  Cost'  Size  and  focal  distance ?  Comparative  size ?) 


2J8 


P.EFRACTING  TELESCOPES. 


THE    PHILADELPHIA   REFRACTOB.* 

525.  This  instrument  is  located  in  the  Observatory  of  the  High- 
School  of  Philadelphia.  Its  focal  length  is  eight  feet,  and  its 
aperture  six  inches.  It  was  made  by  Merz  &  Mahler,  of  Munich, 
and  cost  $2,200. 

*  We  are  indebted  to  the  courtesy  of  Messrs.  Harper  Brothers,  of  New  York,  foi 
copies  of  several  of  theoe  cuts  from  their  Monthly  Magazine  for  June,  1856. 

K.3.  The  PLilaJoiphiaref-actor?    Sizo?    By  whom  mid*  ?    CoutV 


ASTROMOMY. 


220 


COLLEGE  EEST1ACTOB. 


526.  This  instrument  has  a  focal  length  of  sixteen  feet,  with 
an  object-glass  thirteen-and-a-h«ilf  inches  in  diameter.  Its  focal 
length  is  therefore  about  four  fret  less  than  is  usual  in  the  Mu- 
nich instruments  of  the  same  aperture.  The  flint  and  crown 
glass  discs  for  it  were  imported  from  Germany,  and  the  instru- 
ment was  made  by  Messrs.  Spencer  <fc  Katon,  of  Canastota,  N.  Y., 
at  a  cost  of  $10,000.  It  is  reported  to  b<3  a  very  superior  tele- 
scope, and,  in  workmanship,  is  regirdod  as  fully  equal  to  the 
Munich  instruments. 


526.  Sizo  of  the  Hamilton  Cohege  telescope?    \\Tu.t  oe-nf'antv 
whom  mode?    Cost? 


V  length?    By 


ASTRONOMY. 


THE  EQTTATOBIAL  KEFBACTOB  AT  ALBANY,  3T.  Y. 

527.  This  superb  instrument  is  mounted  in  the  Dudley  Ob- 
servatory, at  Albany,  and  is  one  of  the  most  important  instru- 
ments in  America.  Its  focal  length  is  15  feet  2  inches.  The 
object-glass,  made  by  the  late  Henry  Fitz,  of  New  York,  is  13 
inches  clear  aperture,  and  the  tube  is  of  mahogany,  constructed 
by  glueing  together  strips  of  about  an  inch  in  width.  A  finder, 
or  small  telescope  for  finding  objects,  is  seen  attached  to  tho 
]ower  end  of  the  large  instrument. 

537.  Where  located?    Size?    By  whom  made?    What  said  01' tube ?    Finder? 


REFFxACTIXG    TELESCOPES. 


231 


THE  GREAT   EQUATORIAL  REFRACTOR  AT   CAMBRIDGE     MASS. 

528.  This  is  probably  the  best  instrument  in  the  United  States. 
Its  object-glass  is  15  inches  in  diarmter,  with  a  focal  length  of 
22  feet  6  inches.  It  has  eighteen  different  powers,  ranging  from 
103  to  2,000.  It  was  made  by  Merz  &  Mahler,  of  Munich,  Ba- 
varia, and  cost  $19,842. 

The  cut  shows  the  opening  in  the  revolving  dome  of  the  observatory,  and  the  observer 
In  his  chair  at  the  eye-piece. 


528.  Comparative  value ?    Size?    Magnifying  powers?    By  whom  made?    Costoftho 
liiEtrument  ? 


ASTRONOMY 


THE   GKEAT   CRAIG  TELESCOPE,   WANDSWORTH   COMMON,   NEAB   LONDON. 

529.  This  is  the  largest  refracting  telescope  ever  con 
Btructed.  The  object-glass  is  two  feet  in  diameter,  with 
a  focal  distance  of  76  feet.  The  tube  is  of  heavy  sheet 
iron,  and  shaped  somewhat  like  a  cigar.  It  is  13  feet  in 
circumference  in  the  largest  place,  and  weighs  about 
three  tons. 

This  telescope  is  suspended  from  a  brick  tower  65  feet  high,  15  feet  in  diameter,  and 
weighing  220  tons.  The  top  of  the  tower,  from  which  the  telescope  is  suspended,  re- 
volves; and  by  a  chain  running  over  pulleys,  and  a  weight  and  windlass,  it  is  balanced, 
and  raised  or  lowered.  The  lower  end  rests  on  a  small  carriase,  that  runs  npon  a  circu- 
»ir  railroad  around  the  tower,  at  the  distance  of  52  feet  from  its  center.  By  these 
•neans  it  i«  directed  to  almost  any  point  in  the  heavens.  It  is  called  the  "  Craig"  tele- 
Bcop«,  in  honor  of  Rev.  Mr.  Craig,  under  whose  direction,  and  at  whose  expend,  it  was 
coustructed.  It  is  located  at  "Wandsworth  Common,  near  London. 


629.  Describe  the  Craig  telescope.    Object-glass  ?— focal  distance  ?    Tube  ? 
(How  mounted  I     Why  called  "  Craig"  telescope  ?    Where  located  ?) 


TRANSIT   INSTRUMENTS.  223 


A  TRANSIT   INSTRUMENT. 


530.  A  Transit  Instrument  is  a  telescope  used  for  observing 
tlie  transits  of  celestial  objects  across  the  meridian,  for  the  pur- 
pose of  determining  differences  of  right'  ascension,  or  obtaining 
correct  time.  They  are  usually  from  six  to  ten  feet  long,  and 
are  mounted  upon  a  horizontal  axis,  between  two  abutments  of 
mason-work ;  so  that  the  instrument,  when  horizontal,  will  point 
exactly  to  the  south.  It  will  then  take  objects  in  the  heavens, 
when  they  are  exactly  on  the  meridian. 

The  Transit  Instrument  and  Mural  Circle  have  been  combined 
in  one  instrument,  called  a  Meridian  Circle,  as  shown  on  a  sub- 
sequent page. 

Let  A  D  in  the  cut  represent  the  telescope,  and  E  and  W  the  east  and  west  abutments, 
between  which  it  is  placed.  On  the  left  is  seen,  attached  to  the  mason  work,  a  graduated 
circle;  and  on  the  eastern  end  of  the  axis  of  the  telescope  is  seen  an  arm,  «,,  extending 
to  the  circle,  as  an  index.  Now,  suppose  the  index  n  to  be  at  o,  in  the  upper  part  of  the 
circle,  when  the  telescope  is  horizontal;  then  if  the  meridian  altitude  of  the  object  to  bo 
taken  is  10",  the  index  must  be  moved  10"  from  0,  as  the  degrees  on  the  circle  and  the 
altitude  of  the  object  will  correspond. 


580  "What  is  a  transit  instrument ?  Size?  How  mounted?  Describe  parts  as  shown 
in  the  cut.  How  wt  the  instrument  for  tho  altitude  of  a  st<or?  What  combination 
tpokou  of? 


234 


ASTRONOMY. 


531.  An  Astronomical  Clock  is  a  clock  adapted  to  keep 
exact  sidereal  time  (136). 

Taking  tlie  vernal  equinox  in  the  heavens  as  the  zero  point,  and  reckoning  24  houre 
ostwuru  to  the  same  point  again,  the  time — reckoning  15°  to  an  hour — when  an  object 
rosses  the  meridian,  will  always  represent  the  right  ascension  of  the  object  Hence 
•ght  owension  is  usually  given  in  hours,  minutes,  and  seconds;  or  in  time  by  the 
jtroii(  uiieal  clock,  set  by  the  vernal  equinox. 


TUB    MURAL   CIRCLK. 


532.  A  Mural  Circle  is  a  large  graduated  circle,  with 
a  telescope  crossing  its  center,  used  for  the  measurement 
of  the  altitudes  and  zenith  distances  of  the  heavenly 
bodies,  at  the  instant  of  their  crossing  the  meridian 
They  are  usually  fixed  upon  a  horizontal  axis,  that  turn? 
in  a  socket  firmly  fixed  in  a  north  and  south  wall.  The 
degrees,  minutes,  and  seconds  on  the  circle  are  read  b}' 
means  of  microscopes,  and  indicate  the  altitude  of  the 
object. 

In  the  cut.  A  is  a  reading  microscope,  and  B  CD  E  the  wall  to  which  the  circle  is  at 
fetched.  The  telescope  would  denote  an  altitude  of  about  40°,  which  would  leave  5(P  ac 
the  zenith  distance. 


631  .  An  astronomical  clock  1    How  set  I 
objects  '! 

533.  Describe  a  mural  circle  *    Its  uses  1    How  mounted  \ 
altitude  and  zenith  distance  by  it  ?) 


How  indicate  right  ascension  01 
(How  ascertain 


ASTRONOMY. 


235 


TBAN8IT  INSTRUMENT,  -WABniNOTOX,  ».  O. 

533.  This  instrument  is  located  in  the  National  Observatory, 
at  Washington,  D.  C.  It  is  mounted  upon  piers  of  granite,  which 
rest  firmly  apon  a  foundation  of  stone,  extending  ten  feet  below 
the  sirface  of  the  ground.  The  object-glass  was  furnished  by 
Merz  &  Mahler,  and  the  instrument  was  constructed  by  Ertel  & 
Son,  Munich.  The  entire  cost  was  $1,480. 

f>33.  Where  located?    How  mounted?    By  whom  ma<i??    Coot? 


236 


TRANSIT    INSTRUMENTS. 


MERIDIAN   CIRCLE  AT  ALBANY,  N.  T. 

534.  This  is  a  superior  transit  instrument,  with  a  mural  circle 
attached.  It  is  located  in  the  east  wing  of  the  Dudley  Ob&erva- 
tory,  at  Albany,  N.  Y.,  an-d  rests  upon  piers  of  Lockport  lime- 
stone, which  rest  upon  a  bed  of  sand  and  gravel,  some  ten  feet 
below  the  floor  of  the  cellar.  Taken  as  a  whole,  it  is  probably 
the  best  transit  instrument  in  the  United  States. 

1.  A  Mural,  Circle  is  a  larsre  graduated  circle,  with  a  telescope  crossing  Its  center,  used 
for  the  measuTf-ment  of  the  altitudes  and  zenith  distances  of  the  heavenly  bodies,  at  the 
instant  of  their  crossing  the  meridian.  They  are  usually  fixed  upon  a  horizontal  axis, 
that  turns  in  a  socket  "firmly  fixed  in  a  north  and  south  wall.  The  decrees,  minutes, 
and  seconds  on  the  circle  are  read  by  means  of  microscopes,  and  indicate  the  a'.litudo 
of  the  object  The  Mural  Circle  and  a  transit  instrument,  as  now  combined,  are  called. 
a  Meridian  Circle. 


534.  Where  located?    How  mounted?    Comparative  importance?    "What  is  a  Mural 
Oirclef    Use?    Haw  usually  moun tod ?    How  combined?    What  called? 


ASTRONOMY. 


237 


535.  A  Comet  Seeker  is  a 
refracting  telescope  with  a 
large  aperture  and  short  fo- 
cal distance.  As  comets 
cannot  be  found  by  their 
right  ascension  and  declina- 
tion, but  often  have  to  be 
searched  up,  bj  sweeping 
around  the  heavens  with  a 
telescope,  before  they  be- 
come visible  to  the  naked 
eye,  it  is  important  to  have  telescopes  that  wrill  cover 
considerable  space — that  is,  of  wide  aperture  and  short 
focal  distance.  Such  a  telescope  was  made  by  Mr.  Fitz 
for  Miss  Mitchel,  of  Newport,  R.  I. 


REFLECTING   TELESCOPES. 

530.  The  Reflecting  Telescope  is  one  in  which  the  light 
is  converged  to  a  focus  by  means  of  a  concave  metallic 
reflector  or  speculum.  Like  the  Refractors,  they  may 
be  constructed  with  very  little  mounting;  though,  for 
convenience  in  use,  it  is  necessary  to  place  the  reflector 
in  a  tube. 


SIMPLEST   FORM    OF   A    REFLECTING   TELESCOPE. 


In  this  cut,  the  light  A  is  seen  passing  from  the  object  on  the  right,  and  falling  upon 
the  concave  surface  of  the  reflector  at  B,  from  which  it  is  reflected  back  to  a  focus,  and 
enters  the  eye  of  the  observer  at  C  This  telescope  has  no  eye-piece. 

537.  The  focal  distance  of  a  concave  reflector  is  equal 
to  half  the  radius  of  the  sphere  formed  by  the  concave 

535.  What  is  a  comet  seeker  ?    Why  necessary  ? 
53J.  Describe  a  reflecting  telescope.    Simplest  form  . 
537   Focal  distance  ?    (Diagram.) 


238 


DEFLECTING   TELESCOPES. 


durfaee  produced.  Hence  to  grind  a  reflector  for  a  focus 
of  20  feet,  it  will  be  necessary  to  have  the  curve  that  of 
a  circle  whose  radius  is  40  feet. 

FOCAL   DISTANCE    OF   A    CONCAVE    REFLECTOR. 


Here  the  curve  of  the  speculum  B  is  that  of  a  circle,  whose  cen« 
ter  is  C ;  while  the  focus  of  the  speculum  is  at  D,  which  is  ouly 
half  the  distance  from  B  to  C. 


538.  Reflecting  telescopes  are  of  several  kinds — viz., 
the  Gregorian,  the  Newtonian,  the  Gassegranian,  the 
Ilerschelian,  &c.  The  Gregorian  Reflector  has  an  aper- 
ture in  the  center  of  the  speculum,  and  a  small  concave 
mirror  in  the  focus  of  the  speculum,  which  reflects  the 
light  back  through  the  aperture  to  the  eye-piece.  In 
this  way  the  observer  is  enabled  to  face  the  object,  and 
to  direct  the  telescope  toward  it,  as  if  it  were  a  refractor 


GREGORIAN    REFLECTOR. 


Here  the  light  A  falls  upon  the  speculum  at  B,  and  is  reflected  back  to  the  small  mir- 
ror C,  by  which  it  is  thrown  out,  through  the  aperture  in  the  speculum,  to  the  eye  of 
the  observer  at  D.  The  object  is  supposed  to  be  off  on  the  right,  in  the  direction  toward 
which  the  instrument  is  pointed.  It  is  called  a  Gregorian  telescope,  after  Mr.  James 
Gregory,  who  first  suggested  the  construction  of  reflecting  telescopes. 

539.  The  Newtonian  Reflector  is  so  called  after  Sir 
Isaac  Newton,  its  inventor.  Instead  of  a  concave  mir- 
ror in  the  focus  of  the  speculum,  he  placed  a  plane  mir- 


538.  How  many  kinds  of  reflectors  ?    Describe  the  Gregorian.    (Diagram 
Why  o-vlled  Gregorian  ?) 
539    Newtonian  reflectors  ?    (Diagram  and  explanation.) 


ASTRONOMY. 


239 


ror  there,  inclined  so  as  to  reflect  the  light  to  the  side  of 
the  tube,  where  it  was  received  by  the  observer. 


NEWTONIAN    REFLECTOR. 


The  light  from  the  S£KK:nlum  is  here  shown  falling  upon  the  inclined  mirror  in  the 
center,  and  reflected  out  to  the  eye  of  the  observer. 

540.  The  Cassegranian  Reflector  is  so  called  from  M. 
Cassegrain,  its  inventor.      It  resembles  the  Gregorian, 
except  that  the  speculum  placed  in  the  focus  of  the  re- 
flector is  convex,  instead  of  concave. 

541.  The  Ilerschelian  Reflector  differs  from  all  others, 
in  having  no  small  reflector  whatever ;  the  light  being 
reflected  back  to  a  focus  at  the  top  of  the  telescope,  and 
near  the  edge  of  the  tube,  where  the  eye-piece  is  placed, 
and  where  the  observer  sits  looking  into  the  mirror  with 
his  back  to  the  object. 


HERSCHELIAN    TELESCOPE. 


IT  ere  the  concave  spec 
tube,  so  that  the  parallel  rays  A  are  reflected  back  to  the  observer  at  B,  at  the  side  of 
the  instrument,  wliere  the  eye-piece  is  placed. 

542.  The  first  telescope  constructed  upon  this  plan  was 
that  by  Sir  William  llerschel,  in  1782.  This  was  called 
his  20  feet  reflector,  and  was  the  instrument  with  which 
he  made  many  of  his  observations  upon  the  double  staid. 
In  1789,  he  completed  his  forty  feet  reflector,  until 
recently  the  largest  telescope  ever  constructed. 


510.  Cassesrnminn  ?    Difference  ? 

,r.41.  Herschelian?    Where  eye -piece  ?    How  observer  sit  ? 

542.  Flrtet  Uerschelian  telescope ?     What  called?    Next? 


240 


INFLECTING-    TELESCOPES. 


SIK  WILLIAM  HERSCHEL'S  FORTY  FEET  HEFLECTOU. 

543.  The  speculum  of  this  instrument  is  4  feet  in 
diameter,  3^  inches  thick,  and  weighed,  before  being 
ground,  2,118  pounds.  The  tube  is  made  of  sheet  iron 
riveted  together,  and  painted  within  and  without. 

The  length  of  the  tube  is  39  feet  4  inches,  and  its  weight  8,260  pounds.  It  is  elevated 
or  lowered  by  tackles,  attached  to  strong  frame-work ;  and  the  observer,  who  sits  in  a 
chair  at  the  upper  end  of  the  tube,  and  looks  down  into  the  reflector  at  the  bottom,  is 
raised  and  lowered  with  the  instrument.  Three  persons  are  necessary  to  use  this  tele- 
pcope — one  to  observe,  another  to  work  the  tube,  and  a  third  to  note  down  the  observa- 
tions. A  speaking  tube  runs  from  the  observer  to  the  house  where  the  assistants  are  at 
work.  By  this  telescope,  the  sixth  and  seventh  satellites  of  Saturn  were  discovered 
and  it  was  the  chief  instrument  used  by  its  distinguished  owner,  in  making  the  obser- 
vations and  discoveries  which  have  immortalized  his  name,  and  which  have  so  abun- 
dantly eiiriched  and  advanced  the  science  of  astronomy. 

543.  Herschel's  forty  feet  reflector  ?    Size  of  speculum?     Weight?    Tubo 
Length  and  weight  ?    How  mounted  ?    Observer  where  ?    Uset'uJuess  J 


ASTRONOMY 


••  - 


LORD    ROSSE  3    GKKAT    REFLECTING   TELESCOPE. 

oil-.  Tliis  is  the  largest  reflecting  telescope  ever  con- 
structed. The  speculum,  composed  of  copper  and  tin, 
weighed  three  tons  as  it  came  from  the  mould,  and  lost 
about  |th  of  an  inch  in  grinding.  It  is  5J  incjies  thick, 
and  6  feet  in  diameter.  It  was  cast  on  the  13th  of  April, 
1842,  and  was  cooled  gradually  in  an  oven  for  16  weeks, 
to  prevent  its  cracking,  by  a  sudden  t>r  unequal  reduc- 
tion of  the  temperature.  This  speculum  has  a  reflecting 
surface  of  4,071  square  inches.  The  tube  is  made  of 
deal  wood,  one  inch  thick,  land  hooped  with  iron.  Its 
diameter  is  seven  feet,  and  its  length  56. 

The  entire  weight  of  this  telescope  is  twelve  tons. 
It  is  mounted  between  two  north  and  south  walls,  21  feet 
apart,  72  feet  long,  and  48  feet  high.  The  lower  end 
rests  upon  a  universal  hinge.  It  can  be  lowered  to  the 
horizon,  and  raised  to  the  zenith,  and  lowered  northward 
till  it  takes  in  the  Pole  star.  Its  motion  from  east  to 
west  is  limited  to  15  degrees.  This  magnificent  instru- 
ment is  situated  at  Burr  Castle,  Ireland.  It  was  con- 
structed by  the  Earl  of  Rosse,  at  an  expense  of  $60,000 

544.  Lord  Rosse's  telescope  ?    "Weight  of  speculum?    Diameter?    Tliick- 
iH&a  ?    Cooling  9  %Tube  1    Entire  weight  ?    How  mounted!     What  motion  3 

VVhcre  located  ?    Co^, ' 


OBSERVATORIES  AND  TELESCOPES. 


OBSERVATORIES 'AND   TELESCOPES   IN    THE   TTNITKD   STATES. 


CBSEKVAIOEIES. 

THEIK    TELESCOPES. 

Wh-n 
procured. 

Name  of 
utaker. 

Focr.l 
lengih. 

ot$£t*kM. 

Cost. 

Ya'e  College 

1880 
1836 
(1836 
1  1852 
1S37 
1S40 
1S41 
1844 

1846 

1848 
1849 

1850 
1851 
1852 
1846 
1854 
1S53 
1S57  ? 
1S57 
1846 

1S47 
J  1S50 
1  1851 

1S52 

Dollond. 
Lerebours. 
Holcomb. 
A.  Clark. 
Si  nuns. 
Merz. 
Lerebours. 
Merz. 

Siinms. 
Fitz. 
Merz.,: 
Fitz. 

Clark. 
Fitz. 

Spencer. 
Fitz. 

ft.    in. 

10    — 

rr      

10    — 
9    — 
5      6 

8      4 

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15      3 

17    — 
22      6 
9    — 
7      6 
7    — 
10      4 
8      4 
5    — 
7    — 
8      6 
17    — 
15      2 
16    — 
8      4 
7    — 
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8      4 
11    — 
6    — 
5    — 
10    — 
9      6 

inches. 
5 
6 
reflector. 
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$1.000 
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8.500 
1,200 
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6.000 
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2,220 
300 
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1,150 
2.200 

*Yesleyan  University  

"Williams  Collpge  

Hudson  Ohio 

Philadelphia 

West  Point  

Washington      

Dartmouth  College  

Georgetown  

fchelby 

Columbia  (S.  C.)  College  

Columbia  (Mo)  

Friends   Philadelphia       

Amherst  College 

Dudley,  Albany,  N.  Y  

Hamilton  College        

J.  Jackson,  near  Philadelphia.  .. 
Mr.  Longstreet,  Philadelphia  
S.  G.  Gummere,  Burlington,  N.  J. 

II.  Vanarsdale,  Newark,  N.  J  

W.  S.  Van  Duzee,  Buffalo,  N.  Y.  . 
"W  S  Dickie  Elkton  Ky 

D.  Mosnian,  Bangor,  Me  
J.  Campbell,  Now  York         

L.  M.  Rutherford,  New  York  .... 

FOREIGN   OBSERVATORIES — TIIEIB   LATITUDE   AND   LONGITUDE. 


OBSERVATORIES. 

Latitude. 

Lonpritud 

e  iii  Timo. 

Altona 

53 

54 
52 
50 
52 
83 
55 
58 
58 
55 
51 
51 
54 
48 
38 
48 
50 
41 
45 
48 

82 
21 
80 
51 
12 
56 
40 
22 
23 
57 
31 
28 
42 
8 
6 
50 
56 
53 
4 
12 

45 
12.7 
16.7 
10.7 
51.8 
8 
53 
47.1 
13 
23.2 
47.9 
38.2 
50.4 
45 
44 
13 
29.7 
54 
6 
85.5 

N. 
N. 
N. 
N. 
N. 
S. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 
N. 

1) 
0 
0 
0 

0 
0 

1 

0 

1 

0 
0 
0 
0 

1 

0 
0 
0 

g 

0 
0 

1 

m. 
39 
26 
53 
17 
0 
13 
50 
46 
25 
12 
39 
0 
22 
46 
53 
9 
1 
49 
30 
5 

46'.  2 
35.5 
34.9 
27.2 
23.5 
56.0 
19.3 
54.6 
22 
43.0 
46.8 
0.0 
0.4 
25.4 
25.5 
21.5 
13.5 
54.7 
48.4 
32.6 

E. 
W. 
E. 

E. 
E. 
E. 
E. 
E. 

w! 

E. 

E. 
E. 
E. 
E. 
E. 
E. 
E. 
E. 

Berlin  

Brussels  

Cape  of  Good  Hope 

Copenhagen  

Dorpat  

Dublin  

Edinburgh.  

Gottiniren  

Konigsberg  

Munich 

Palermo  

Paris              

Turin  

Vienna  

Public  observatories  in  this  country?     Largest  telescope  ?       Table?        Private 
observatories — names  ?    Telescopes — by  whom  mostly  made  ?    What  other  table  ? 


PARALLAX    OF   TEE    HEAVENLY    BODIES. 


545.  Parallax  is  the  difference  between  the  altitude  of 
any  celestial  object  seen  from  the  earth's  surface,  and  tho 
altitude  of  the  same  object  seen  at  the  same  time  from 
the  earth's  center;  or  it  is  the  angle  under  which  the 
semi-diameter  of  the  earth  would  appeal-,  as  seen  from 
the  object. 

The'  true  place  of  a  celestial  body  is  that  point  of  the 
heavens  in  which  it  would  be  seen  by  an  eye  placed  at 
the  center  of  the  earth.  The  apparent  place  is  that  point 
of  the  heavens  where  the  body  is  seen  from  the  surface 
of  the  earth.  The  parallax  of  a  heavenly  body  is  great- 
est when  in  the  horizon,  and  is  thence  called  the  hori- 
zontal parallax.  Parallax  decreases  as  the  body  ascends 
toward  the  zenith,  at  which  place 
it  is  nothing:. 


PARALLAX  OF  THE  PLAXKT8. 

F 


The  adjoining  cut  will  afford  a  sufficient  illustration. 
When  the  observer,  standing  upon  the  earth  at  A, 
vieivs  the  object  at  B,  it  appears  to  be  at  C,  when,  at 
the  same  time,  if  viewed  from  the  center  of  the  earth, 
it  would  appear  to  be  atD.  The  parallax  is  the  angle 
B  G  D  or  A  B  E,  which  is  the  difference  between  the 
altitude  of  the  object  B,  when  seen  from  the  earth's 
surface,  and  when  seen  from  her  center.  It  is  also 
the  angle  under  which  the  semi-diameter  of  the  earth, 
AE,  is  seen  from  the  object  B. 

As  the  object  advances  from  the  horizon  to  the  ze- 
nith, the  parallax  is  seen  gradually  to  diminish,  till  at 
F  it  has  no  parallax,  or  its  apparent  and  true  place  are 
the  same. 

This  diagram  will  also  show  why  objects  nearest 
the  earth  have  the  greatest  parallax,  and  those  most 

distant  the  least ;  why  the  moon,  the  nearest  of  all  the  heavenly  bodies,  has  the  greatest 
parallax;  while  the  "fixed  stars,  from  their  immense  distance,  have  no  appreciable 
horizontal  parallax— the  semi-diaineter  of  the  earth,  at  such  a  distance,  being  no  more 
than  a  point 

546.  As  the  effect  of  parallax  on  a  heavenly  body  is 
to  depress  it  "below  its  true  place,  it  must  necessarily  affect 
its  right  ascension  and  declination,  its  latitude  and  longi- 
tude. On  this  account,  the  parallax  of  the  sun  and 
moon  must  be  added  to  their  apparent  altitude,  in  order 
to  obtain  their  true  altitude. 

The  true  altitude  of  the  sun  and  moon,  except  when  in  the  zenith,  is  always  affected, 
more  or  less,  both  by  parallax  and  refraction,  but  always  in  a  contrary  manner.  Hence 
the  mariner,  in  finding  the  latitude  at  sea,  h  ways  adds  the  parallax,  and  »ubtracta  the 
refraction,  to  and  from  the  sun's  observed  altitude,  in  order  to  obtain  the  true  altitude, 
find  thence  the  latitude. 

545.  Parallax?     True  place  of  a  celestial  body  ?     Apparent?     W lien  par- 
allax greatest  ?     Least?     Called  what,  and  why  2     (Diagram  ?     What  objects 
greatest  parallax?) 

546.  Effect  of  parallax?    How  obtain  true  altitude  ?    (IIo\v  differ  from  ro- 
fhictiou  ?     How  theu  obtain  true  altitude  I) 


244 


547.  The  principles  of  parallax  are  of  great  import- 
ance to  astronomy,  as  they  enable  us  to  determine  the 
distances  of  the  heavenly  bodies  from  the  earth,  the  mag- 
nitudes of  the  planets,  and  the  dimensions  of  their  or- 
bits. 

The  sun's  horizontal  parallax  being  accurately  known, 
the  earth's  distance  from  the  sun  becomes  known ;  and 
the  earth's  distance  from  the  sun  being  known,  that  of 
all  the  planets  may  be  known  also,  because  wre  know  the 
exact  periods  of  their  sidereal  revolutions,  and,  according 
to  the  third  law  of  Kepler,  the  squares  of  the  times  of 
their  revolutions  are  proportional  to  the  cubes  of  their 
mean  distances.  Hence,  the  first  great  desideratum  in 
astronomy,  where  measure  and  magnitude  are  concerned, 
is  the  determination  of  the  true  parallax. 

At  a  council  of  astronomers  assembled  in  London  some  yenrs  since,  from  the  most 
learned  nations  in  Europe,  the  sun's  mean  horixontal  parallax  was  settled,  as  tlie  result 
of  their  united  observations,  at  0°  0'  8".57T6.  Now  the  value  of  radius,  expressed  like- 
wise in  seconds,  is  2oG264".8;  and  tins  divided  by  8".577(>,  gives  24047  for  the  distance 
of  the  sun  from  the  earth,  in  semi-diameters  of  the  latter.  If  we  take  the  «iu<itori«l 
Bemi-dianieter  of  the  earth  as  sanctioned  by  the  same  tribunal,  at  (7924-7-2=)  8962  miles, 
we  shall  have  24047X3962=95,273,869  miles  for  the  sun's  true  distance. 

548.  The  change  in  the  apparent  position  of  the  fixed 
stars,  caused  by  the  change  of  the  earth's  place  in  her 
revolution  around  the  sun,  is  called  their  annual  paral- 
lax.    So  immense  is  their  distance,  that  the  semi-annual 
variation  of  190,000,000  of  miles  in  the  earth's  distance, 
from  all  those  stars  that  lie  in  the  plane  of  her  orbit, 
makes  no  perceptible  difference  in  their  apparent  magni- 
tude or  brightness. 

The  following  cut  will  illustrate  our  meaning: 


B 

Let  A  represent  a  fixed  star  in  the  plane  of  the  enrth's  orbit,  B.  At  C,  the  earth  5a 
190  000  000  of  miles  nearer  the  star  than  it  will  be  at  D  six  months  afterward;  and  yet 
this  semi-annual  variation  of  190,000,000  miles  in  the  distance  of  the  star  is  so  small  a 
fraction  of  the  whole  distance  to  it,  as  neither  to  increase  or  diminish  its  apparent 
brightness, 

547.  Use  of  parallax  ?    Row  employed  ?    (Note  ?) 

548.  What  meant  by  earth's  annual  parallax  ?  Eil'ect  of  variation  of  earth  A 
U.etauuc  on  the  fixed  stars  ?     (Diagram.) 


SnSCELLAJSTIA.  245 


549.  It  is  only  those  stars  that  are  situated  near  the 
axis  of  the  earth's  orbit  whose  parallax  can  be  measured 
at  all.  on  account  of  its  almost  imper- 

ceptible  quantity.  So  distant  are 
they,  that  the  variation  of  190,000,000 
miles  in  the  earth's  place  causes  an 
apparent  change  of  less  than  1'  in 
the  nearest  and  most  favorably  situ- 
ated fixed  star. 

Let  A  represent  the  earth  on  the  1st  of  January,  and  B 
a  star  observed  at  that  time.  Of  course,  its  apparent  place 
in  the  more  distant  heavens  will  be  at  C.  But  in  six 
months  the  earth  will  be  at  D,  and  the  star  B  will  appear 
to  be  at  E.  The  angle  A  B  D  or  0  B  E  will  constitute 
the  parallatic  angle.  In  the  cut,  this  angle  amounts  to 
about  43°,  whereas  the  real  parallax  of  the  stars  is  less 
than  -g^h  of  one  degree,  or  ^^pftb  part  this  amount. 
Lines  approaching  each  other  thus  slowly  would  appear 
parallel ;  and  the  earth's  orbit,  if  filled  with  a  globe  of  fire, 
and  viewed  from  the  fixed  stars,  would  appear  but  a  point  of  light  1'  in  diameter! 

MISCELLANIA. 

550.  The  Atmosphere  is  an  elastic  gas,  which  sur- 
rounds the  earth  on  every  side.     It  is  supposed  to  be 
from  40  to  60  miles  in  Light,  growing  more  rare  as  we 
ascend,  and  is  kept  around  the  earth  by  attraction. 

551.  Wind  is  air  put  in  motion  by  heat,  causing  bodies 
of  air  to  rise  from  the  earth's  surface,  and  other  air  to 
rush  in  to  supply  its  place.     The  velocity  of  the  wind 
ranges  from  5  to  100  miles  an  hour. 

552.  Clouds  are  collections  of  vapor  suspended  in  the 
air.     They  range  from  two  miles  to  half  a  mile  in  hight, 
according  to  their  density  and  weight.     They  serve  to 
screen  us  from  the  oppressive  heat  of  the  sun,  and  to 
convey  water  from  the  rivers  and  oceans,  and  pour  it 
down  in  showers  upon  the  earth. 

553.  Rain  is  water  condensed,  or  collected  into  drops 
by  attraction,  and  falling  from  the  clouds.     Hail  is  drops 

549.  "What  stars  have  perceptible  parallax?    Amount?     (Diagram,  arid 
explain.) 

550.  What  is  the  atmosphere  ?    Extent  ?    How  kept  around  tho  ourth  ? 

551.  Wind  ?    How  put  in  motion  ?    Velocity  * 

552.  Clouds?    "Uses? 

553.  Rain?     ILiil  ?    Snow? 


246  ASTRONOMY. 


of  ruin  frozen  on  its  way  to  the  earth  ;  and  Snow  is  par- 
ticles of  clouds  frozen  before  being  condensed  into  drops, 

554.  Lightning  is  electricity  passing  from  one  cloud 
to  another,  or  between  the  clouds  and  the  earth  ;  and 
Thunder  is  the  sudden  shock  given  to  the  atmosphere 
by  the  passage  of  the  electricity  through  it. 

555.  The  Aurora  Borealis,  or  Northern  Light,  is  a 
reddish  'insteady  light  sometimes  seen  in  the  north.     It 
is  supposed  to  be  caused  by  electricity  passing  through 
the  upper  regions  of  the  atmosphere,  about  the  North 
Pole. 

556.  "  Shooting  Stars"  are  meteors  that  shoot  down- 
ward toward   the   earth,  like   stars   falling  from   their 
spheres.     They  are  usually  seen  one  at  a  time,  and  only 
in  the  night,  but  sometimes  fall  in  showers,  and  no  doubt 
fall  in  the  day  time,  though  invisible. 

From  2  o'clock  in  the  morning,  November  13,  1883,  till  daylight,  the  whole  heavens 
were  filled  with  these  fiery  particles  and  streaks  of  light  darting  downward  from  the 
sky.  These  meteors,  no  doubt,  come  from  regions  beyond  the  limits  of  the  atmosphere, 
and  arc  isrnited  by  their  rapid  passage  through  it.  Their  origin  and  nature  are  as  yet 
matters  of  inquiry  and  speculation. 

557.  Aerolites,  or  Meteoric  Stones,  are  masses  of  stone 
or  iron  that  have  fallen  from  the  sky  at  various  periods, 
and  on  almost  every  part  of  the  globe.     They  are  often 
found  after  the  explosion  of  large  meteors,  sometimes 
while  they  are  yet  hot. 

A  large  meteor  exploded  over  Cabarras  county,  North  Carolina,  a  few  years  since, 
several  pieces  of  which  were  picked  up  the  next  day.  One  piece,  weighing  19  Ibs.,  had 
struck  a  large  pine  tree  lying  on  the  ground,  and  had  gone  through  it,  and  into  the 
earth,  to  the  depth  of  three  feet.  In  some  cases,  large  masses  of  iron  have  fallen.  In 
December,  1795,  a  stone  weighing  51  Ibs.  fell  in  Yorkshire,  England.  The  writer  has  a 
piece  of  an  aerolite  that  weighed  90  Ibs.,  that  fell  in  New  Jersey.  A  large  mass  of 
meteoric  iron  may  be  seen  in  the  museum  of  Yale  College. 

554.  Lightning  and  thunder  ? 

555.  Aurora  Borealis  ? 

556.  "Shooting  stars?"    How  seen?    (What  shower  mentioned?    D& 
ianoe  from  which  they  come  ?) 

557.  Aerolites  ?    (What  instances  of  their  full  cited  ?) 


TABLE    OF   THE   A3TEROIDS. 


247 


TABLE   OF  THE   ASTEROIDS. 


558.  The  following  table  comprises  the  names,  distances, 
periods,  etc.,  of  the  Asteroids,  so  far  as  known.  They  are 
placed  in  the  order  of  their  discovery. 


No         Names. 

Distance  from 
the   Sun  in 
Miles. 

Periodic 
time   in 
Day*. 

Time  of 
discovery. 

By  whom 
discovered. 

Whrro 
discove.c-J. 

1.  Ceres  
2   Pallas 

202.764.110 
263,186.670 
253.5-24.410 
2-24,827.205 
244.767,500 
230.414,710 
22fi.fis3.965 
209,131,670 
226,644.350 
21)9.190,435 
2:-,-2,!)95,S60 
221.617.045 
244,684,375 
245,989,960 
251,197,100 
277,661,440 
235.002,450 
218,125.700 
231,929,960 
223.891,670 
231.365.945 
237.080,005 
249.738.280 
299.244.965 
228,100,700 
252.327,505 
222.993,975 
268,641,815 
242,712,270 
224.598,905 
299,835,010 
245,958,705 
272,372,125 
255.388,690 
288,216,755 
261.126.975 
255,981,165 
260.270,075 
263,091,765 
215,879,060 
228,032,015 
231,219,455 
209.364,610 
230,886,670 
260,568.660 
241,296,960 
273,641,325 

1,680 
1,6S4 
1,592 
1,3-25 
1,511 
1.380 
1.346 
1,193 
1.346 
2,041 
1.403 
1,301 
1,510 
1.522 
1.570 
1,825 
1,421 
1,271 
1,393 
1,366 
1,388 
1,440 
1.557 
2,042 
1,359 
1,581 
1,314 
1,689 
1,492 
1,328 
2,048 
1,522 
1,773 
1,610 
1,880 
1,665 
1,568 
1,(;56 
1,683 
1,247 
1,358 
1.SS7 
1.195 
1.3S4 
1.659 
1,479 
1,786 

Jan.        1.  1801 
March  28,  1802 
Sept      1,  1804 
March  29,  1807 
Dec.       8,  1845 
July      1,  1847 
Aug.    13.  1847 
Oct.      18,  1847 
April    25,1848 
April    12,  1849 
May     13,  1850 
Sept.    13,  1850 
Nov.      2,  1850 
May     20,  1S51 
July     29,  1851 
March  17,  1852 
April    17,  1S52 
June    24,  1852 
Aug.     22,  1852 
Sept.    19,  1S52 
Nov.    15,  1852 
Nov.    16,  185-2 
Dec.     15,  1852 
April      5,  1853 
April      6,  1853 
May       5,  1853 
Nov.      8.  1S53 
March    1,  1854 
March    1,  1854 
July     22,  1854 
Sept.      1,  1854 
Oct.      26,  1854 
Oct.      28,  1854 
April    15,  1855 
April   19,  1855 
Oct.       5,  1855 
Oct       5,  1855 
Jan.     12,  1S56 
Feb.       8,  1856 
March  81,  1856 
May     23,  1856 
May     23,  1856 
April   15,  1857 
May     27,  1857 
June    27,  1857 
Aug.    16,  1857 
Sept.    15,  1857 

Piazzi  
Dr.  Olbers.. 
Harding.... 
Dr.  Olbers. 
Hen  eke  
Hen  eke  
Hind 

Palermo. 
Bremen. 
Lilienthal. 
Bremen. 
Drk'sden. 
Driesden. 
London. 
London. 
Markvee. 
Naples. 
Naples. 
London. 
Naples. 
London. 
Naples. 
Naples. 
Bilk. 
London. 
London. 
Marseilles. 
Paris. 
London. 
London. 
Naples. 
Marseilles. 
Bilk. 
London. 
Bilk. 
London. 
London. 
Washington,  D.  C, 
Paris. 
Paris. 
Paris. 
Bilk. 
Paris. 
Bilk. 
Paris. 
Paris. 
Paris. 
Paris. 
Oxford. 
Oxford. 
Paris. 
Paris. 
Oxford. 
Bilk. 

8.  Juno  

4.  Vesta  

5.  Astnea.  
6   Hebe 

7.  Iris  
8.  PMora 

Hind  
Graham  
De  Gasparis 
De  Gasparis 
Hind  
De  Gasparis 
Hind  
De  Gasparis 
De  Gasparis 
Luther  
Hind  

9.  Metis  
10.  Hygeia  
11.  Parthenope  . 
12.  Clio          .    .. 

13.  Egeria  
14.  Irene    

15.  Eunomia  ... 
16.  Psvcho  
17.  Thetis  
IS.  Melpomena 
19.  Fortuna  
20.  Massilia  
21.  Lutetia  
22.  Calliope  
23.  Thalia  
24.  Themis  
25.  Phoccea  
26.  Proserpine.. 
27.  Euterpe  
28.  Bellona  
29.  Amphitrite  . 
8.0.  Urania  
81.  Euphrosyne 
82.  Pomona    ... 
33.  Polymnia... 
34.  Circe  
35.  Leucothea  .  . 
86.  Atalanta  
37.  Fides  
38.  Leda  
89.  Laititia  
40.  Harmonia  .. 
41.  Daphne  
42.  Isis    
43.  Ariadne  
44.  Nysa  
45.  Eugenia  
46.  Hestia  
47.  Aglaia  

Hind  
Chacornac.. 
Goldsmidt.. 
Hind 

Hind 

De  Gasparis 
Chacoruac.. 
Luther    
Hind  

Luther  
Marth... 
Hind  

Ferguson  .. 
Goldsmidt  . 
Chacornac.  . 
Chacornac.. 
Luther  
Goldsmidt.. 
Luther  
Chacornac.. 
Chacornac.  . 
Goldsmidt.. 
Goldsmidt.. 
Pogson  
Poison  .   ... 
Go'ldsmidt.. 
Goldsmidt.. 
Pogson  
Luther  

248  ASTRONOMY. 

TABLE  OF  THE   ASTEROIDS.—  Continued. 


No.  Names. 

Distance  from 
the  Sun   in 
MiieB. 

Periodic 
lime   in 
Days. 

T:me  of 
discovery. 

By  wh<  m 
discovered. 

Where 
discovi-red. 

43  Doris 

295,150,275 
293,180,925 
•251.844,430 
225,901,640 
294,330,710 
248.224.930 
25S.S1  1.540 
263.965,195 
245.428.700 
296,942.265 
255;971,895 
257.714,955 
'227,203,995 
285.377.815 
297,430,750 
2-27.654.200 
254,437.170 
3-25.996.965 
252,117,278 
229,421,200 
258.652.510 
290,924,010 
253,602,065 
203.783,740 
201,841,470 
254,435,102 
244,645.135 
251,121.955 
302,955,000 
253,521,413 
262,418,500 
232,294,000 
215.390,742 
263,981,794 
257,814,930 
232,297,428 
225,900,271 
252,117,294 

2,000 
1,980 
1,577 
1.839 
1,992 
1,543 
1,642 
1,692 
1,517 
2,049 
1.615 
1,632 
1,351 
1,902 
2,024 
1,355 
1,601 
2.322 
1,579 
1,371 
1.641 
1,957 
1,594 
1.148 
1,671 
1,589 
1,509 
1.570 
2080 
1.597 
1,677 
1,397 
1.271 
1,693 
1.659 
1.382 
1,324 
1,572 

Sept.  19,1857 
Sept  19,1857 
Oct.  4,  1857 
Jan.  22,  1S5S 
Feb.  4,  1858 
April  4,  1858 
Sept.  11,  1858 
Sept.  11.  1858 
Sept.  9.  1859 
Sept.  22,  1859 
March  24,  1800 
Sent.  12,  I860 
Sept.  15,  I860 
Sept.  19,  1800 
Oct.  10,  I860 
Feb.  10.  1861 
March  2,1861 
March  4,  1861 
April  9,  1861 
April  17,  1861 
April  20,  1861 
April  29,  1861 
May  5,  1861 
May  29,1801 
An?.  13,  1861 
April  7,  186.' 
A  us?.  29,1862 
Sept.  22,1862 
Oct.  21,  1862 
Nov.  12,  1862 
March  15,  1863 
Sept.  14,  1S63 
May  2,  1864 
Sept.  30,1864 
Nov.  27,  1864 
April  26,  1865 
Aug.  25,  1865 
Sept  19,  1865 

joldsmidt.. 
joldsmidt.  . 
Ferguson  .  .  . 
Laurent  .... 
Joldsmidt.. 
Luther  
Goldsuiidt.. 
Searle  
joldsmidt.. 
Luther  
Luther  .... 
Jhacornac.. 
Ferguson  .  .  . 
Groldsmidt.  . 
Foster   .... 
l)e  Gasparis 
1'empel  .... 
Tempo!  
Tuttie  
Pay  son  
Luther  
Sehiaparelii 
Goldsmidt. 
Peters  
Luther.  
Tuttie  
Tern  pel  

Paris. 
Paris. 
Washington,  D.  C. 
Nismes. 
1-aris. 
Bilk. 
Paris. 
Albany,  N.  Y. 
Paris. 
Bilk. 
Bilk. 
Paris. 
Washington,  D.  C. 
Paris. 
Berlin. 
Naples. 
Marseilles. 
Marseilles. 
Cambridge,  Mass. 
Madras. 
Bilk. 
Milan. 
Paris. 
Clinton,  N.  Y. 
Bilk. 
Cambridge,  Mass. 
Marseilles. 
Clinton   N  Y. 

49  Pales 

50.  Virginia.... 
61.  Nemausa  
52.  Europa  
53.  Calypso  
54.  Alexandra  .. 
55.  Pandora  .... 
50.  Melete  

57.  Mnemosyne 
58.  Concordia  .  . 
59.  Olympia  
CO.  Echo  

61.  Danae  
6-2.  Erato  
(53.  Ansonia  
64.  Angelina  .  .  . 
65.  Cybele  
66.  Maja  
67.  Alia  

68  Leto  . 

69.  llcsperia  
70.  Panoprca  
71.  Feronia  
72.  Niobe  
73.  Clytie  

74.  Galatea  
75.  Euridice  
76.  Freia  
77.  Frigga  
78.  Diana  
79.  Eurynome.. 
80.  Sappho  
81.  Terpsichore. 
82.  Alcmene  ... 
83.  Beatrix  
84  Clio  

M.  D.  Arvert 
Peters  
Luther  
Watson  
Pogson  
Tern  pel  
Luther  
De  Gasparis 
Luther  .... 
Peters  

.Copenhagen 
Clinton,  IST.  Y. 
Bilk. 
Ann  Arbor,  Mich. 
Oxford. 
Marseilles. 
Bilk. 
Naples. 
Bilk. 
Clinton,  K  Y. 

85  lo 

PROBLEMS  AND  TABLES.  249 


PROBLEMS  AND   TABLES. 

I. TO    CONVERT    DEGREES,    ETC.,    INTO    TIME. 

RULE  1. — Divide  the  degrees  by  15,  for  hours ;  and  multiply 
the  remainder,  if  any,  by  4,  for  minutes. 

2.  Divide  the  odd  minutes  and  seconds  in  the  same  man- 
ner by  15  for  minutes,  seconds,  &c.,  and  multiply  each  remain- 
der by  4,  for  the  next  lower  denomination. 

EXAMPLE  1.— Convert  32°  34'  45"  into  time. 

Thus,  32°-f.l5  =  2h.     8' 

34   -M5  2     16" 

45   -M5  3 

Ans.     32*  34'  45"  =  2h.  10'  19" 

EXAMPLE  2. — If  it  is  12  o'clock  at  this  place,  what  is  tho 
time  20°  east  of  us  ? 

II. TO    CONVERT    TIME  .INTO    DEGREES,    ETC. 

RULE. — Multiply  the  hours  by  15,  and  to  the  product  add 
one-fourth  of  the  minutes,  seconds,  &c.',  observing  that  every 
minute  of  time  makes  J°,  and  every  second  of  time  £'. 

EXAMPLE  1. — In  2  hours,  10  minutes,  and  19  seconds,  how 
many  degrees? 

Thus,  2h.     10m.     19. 

15 

30° 

Add  10  quarters,  or  \  of  the  min.     2       30' 
Add  19  quarters,  or  \  of  the  sec.  4          45" 


Ans.        32*     34'         45" 

Ex.  2. — When  it  is  12  o'clock  at  Hartford,  it  is  4  hours,  51 
minutes,  and  20  seconds  past  noon  at  Greenwich;  how  many 
degrees  is  Hartford  west  of  Greenwich? 

Thus:  15  times  4  is  60— added  to  1  of  51,  is  72C  45",  and 
this,  increased  by  i  of  20,  is  72°  50'.  Ans. 


250  AST30NOSIY. 


Ex.  3. — The  moment  of  greatest  darkness,  during  the  annu- 
lar eclipse  of  1831,  took  place  at  New  Haven,  10  minutes 
after  1  o'clock.  A  gentleman  reports  that  it  happened  pre- 
cisely at  1,  where  he  observed  it;  and  another,  that  it  was  5 
minutes  after  1  where  he  saw  it :  Query.  How  far  east  or  west 
were  these  gentlemen  from  each  other,  and  how  many  degrees 
from  New  Haven  ? 


RULE. — As  the  Sun's  horizontal  parallax  is  to  radius,  so  is 
the  semi-diameter  of  the  Earth  to  its  distance  from  the  Sun. 

By  Logarithms. — As  tangent  of  the  Sun's  horizontal  par- 
allax is  to  radius,  so  is  the  Earth's  semi-diameter  to  her  mean 
distance  from  the  Sun. 

8--.57T6:  206264".8:  :3962:  95,273,869  miles. 

By  Lo(iaritli.m8. 

As  tansrnt  of  the  Sun's  horizontal  parallax,  S".5776=  5.61S9407 
Is  to  radius,  or  90°,  =10.000000:) 

So  is  the  Eni-th's  semi-diameter,  3962=  3.5979143 

To  the  Earth's  distance,  95,273,869=  7.97«9733 

IV. TO    FIND    THE     DISTANCE    OF    ANY    PLANET    FROM    THE     SUN, 

THAT    OF    THE    EARTH    BEING    KNOWN. 

RULE. — Divide  the  square  o'f  the  planet's  sidereal  revolution 
round  the  Sun,  by  the  square  of  the  Earth's  sidereal  revolution, 
and  multiply  the  cube  root  of  the  quotient  by  the  Earth's 
mean  distance  from  the  Sun. 

By  Logarithms. — From  twice  the  logarithm  of  the  planet's 
sidereal  revolution,  subtract  twice  the  logarithm  of  the  Earth's 
sidereal  revolution,  and  to  one-third  of  the  remainder  add  the 
logarithm  of  the  Earth's  mean  distance  from  the  Sun. 

EXAMPLE. — Required  Mercury's  mean  distance  from  the  Sun,  that  of  the  Earth 
being  95,273,869  miles. 

Mercury's  sidereal  revolution  is  87.969253  days,  or  7600543".S912 :  the  Earth's 
sidereal  revolution  is  365.256374417  days,  or 

8155S151".5  7600543.9 

81558151  ".5  7600543.9 


995916962096952.25  by  which  divide   5776S267575827.21 

and  the  quotient  will  be  0.052005106713292,  the  cube  root  of  which  is  0.3870977,  ami 
this  multiplied  by  94,881,891  gives  36,727,607  miles,  for  Mercury's  distance  from  the 
Bun.  This  problem  may  be  performed  by  logarithms  in  as  many  minutes  as  the 
former  method  requires  hours. 

Mercury's  Sid.  Re  v.  7600548".9  log.  =  6.8808447  x  2  13.761  6894 

Earth's  Sid.  Kev.     81558151".    log.  =7.4991302x2  14.99S2604 


1.5S78097 
Add  log.  of  the  Earth's  mean  distance,  7.9789738 

Mercury's  distance,  30,880,422.  Ana.  7.5667835 


PROBLEMS    A>TO    TABLES.  25  i 


V. TO    FIND    THE    HOURLY    MOTION    OF  A  PLANET    IN    ITS    ORBIT. 

RULE. — Multiply  the  planet's  mean  distance  from  the  Sun 
by  6.2831853,  and  divide  the  product  by  the  time  of  the 
planet's  sidereal  revolution,  expressed  in-  hours,  and  the  deci 
mals  of  an  hour. 

By  Logarithms. — Add  0.7981799  to  the  logarithm  of  the 
planet's  mean  distance  from  the  Sun,  and  from  the  sum  subtract 
the  logarithm  of  the  planet's  revolution,  expressed  in  hours. 

EXAMPLE.— Required  the  Earth's  hourly  motion  in  its  orbit 

Log.  of  Earth's  distance=Y.97S073S  +  0.79S1799=  8.7771587 

Subtract  loff.  of  Earth's  revolution  8.9428090 

Gives  Earth's  horary  motion,  63,288  miles,  4.8343447 

VI. TO    FIND    THE    HOURLY    MOTION    OF  A    PLANET    ON    ITS    AXIS. 

RULE. — Multiply  the  diameter  of  the  given  planet  by  3.141 59, 
and  divide  the  product  by  the  period  of  its  diurnal  rotation. 

By  Logarithms. — Add  4.0534524  to  the  logarithm  of  the 
planet's  diameter,  and  from  the  sum  subtract  the  logarithm  of 
its  diurnal  rotation,  expressed  in  seconds. 

Earth's  diameter,  7924  lo<r.  =  3.S9S9445 

Add  log.  of  8000"  +  log.  of  3.14159  =  4.0584524 

7.9523%'j 
Subtract  log.  diurnal  rotation,  23h.  5G'  4".09  =  4.9858-2 G3 

Ans.    1040.09  miles  =  8.0170706 

VII. TO    FIND    THE    RELATIVE    MAGNITUDE    OF    THE     PLANETS. 

RULE. — Divide  the  cube  of  the  diameter  of  the  larger  planet 
by  the  cube  of  the  diameter  of  the  less. 

By  Logarithms. — From  three  times  the  logarithm  of  the 
larger,  subtract  three  times  the  logarithm  of  the  less. 

EXAMPLE. — How  much  does  the  size  of  the  Earth  exceed  that  of  the.  Moon  ? 
Earth's  diameter,  7912  log.  3.89S2S63  x  3=  11.69485S9 

Moon's  diameter,  2160  lo<;.  3.3343376  x  3=  10.0030128 

The  Earth  exceeds  the  Moon,  49.1865  times.    Ans.  1.6918461 

In  this  example,  7912  miles  is  assumed  as  the  mean  between  the  Earth's  equato 
rial  and  polar  diameter  :  the  former  being  7924,  and  the  latter  7898  miles. 

VIII. TO     FIND     THE     PROPORTION    OF    SOLAR    LIGHT    AND    HEAT 

AT    EACH    OF    THE    PLANETS. 

RULE. — Divide  the  square  of  the  planet's  greater  distance 
from  the  Sun,  by  the  square  of  the  less. — Or,  subtract  twice 
the  logarithm  of  the  greater  distance  from  twice  the  logarithm 
of  the  less. 


252 


EXAMPLE. — How  raucli  greater  is  tlie  Sun's  light  and  lieat  at 
Mercury,  than  at  the  Earth  1 


Log.  of  Earth's  distance 

"      of  Mercury's 
Ans.  6.6736  time's  greater; 


7.97S973S  x  2=15.95794T6 

7.5607959  x  2=15.18.35913 

0.824-3558 


IX. TO    FIND    THE    CIRCUMFERENCE    OF    THE    PLANETS. 

RULE. — Multiply  the  diameter  of  the  planet  by  3.14159,  or, 
add  the  logarithm  of  the  planet's  diameter  to  0.4971499. 

X. TO    FIND    THE    CIRCUMFERENCE    OF    THE    PLANETARY  ORBITS. 

RULE. — Multiply  the  planet's  mean  distance  from  the  Snu 
by  6.2831853;  or,  to  the  logarithm  of  the  planet's  mean  dis- 
tance add  0.7981799,  and  the  sum  will  be  the  logarithm  of  the 
answer. 

£1. TO     FIND     IN     WHAT     TIME     ANY     OF     THE     PLANETS    WOULD 

FALL    TO    THE    SUN,    IF    LEFT    TO    THE    FORCE    OF    GRAVITATION 

ALONE. 

RULE. — Multiply  the  time  of  the  planet's  sidereal  revolution 
by  0.176776;  the  result  will  be  the  answer. 

By  Logarithms. — From  the  logarithm  of  the  planet's 
sidereal  revolution,  subtract  0.7525750,  and  the  remainder 
will  be  the  logarithm  of  the  answer,  in  the  same  denomination 
as  the  sidereal  revolution. 

Required  the  times,  respectively,  in  which  the  several  planets  would  fall  to  the 
Sun  by  the  force  of  gravity. 


Planets  would  fall  to  the  Sun. 

Days.    H.    M.    S. 

Logarithms. 

Mercury, 
Venus, 
Earth, 
Mars, 
Jupiter, 
Saturn, 
Herschel, 
Moon  to  the  Earth, 

15        13    13    16 
89        17    19    22 
64        13    38    55 
121        10    86      3 
265        21    33    35 
1901        23    24      4 
5424        16    52      1 
4        19    54    57 

6.12S26S6 
6.5355424 
6.7465357 
7.0208S17 
7.S206S49 
8.21571S6 
8.6708S97 
5.6204459 

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